text: /**
   * @license Copyright (c) 2003-2021, CKSource - Frederico Knabben. All rights reserved.
   * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
   */
  
  /**
   * @module engine/model/schema
   */
  
  import CKEditorError from '@ckeditor/ckeditor5-utils/src/ckeditorerror';
  import ObservableMixin from '@ckeditor/ckeditor5-utils/src/observablemixin';
  import mix from '@ckeditor/ckeditor5-utils/src/mix';
  
  import Range from './range';
  import Position from './position';
  import Element from './element';
  import Text from './text';
  import TreeWalker from './treewalker';
  
  /**
   * The model's schema. It defines allowed and disallowed structures of nodes as well as nodes' attributes.
   * The schema is usually defined by features and based on them the editing framework and features
   * make decisions how to change and process the model.
   *
   * The instance of schema is available in {@link module:engine/model/model~Model#schema `editor.model.schema`}.
   *
   * Read more about the schema in:
   *
   * * {@glink framework/guides/architecture/editing-engine#schema Schema} section of the
   * {@glink framework/guides/architecture/editing-engine Introduction to the Editing engine architecture}.
   * * {@glink framework/guides/deep-dive/schema Schema deep dive} guide.
   *
   * @mixes module:utils/observablemixin~ObservableMixin
   */
  export default class Schema {
  	/**
  	 * Creates schema instance.
  	 */
  	constructor() {
  		this._sourceDefinitions = {};
  
  		/**
  		 * A dictionary containing attribute properties.
  		 *
  		 * @private
  		 * @member {Object.<String,String>}
  		 */
  		this._attributeProperties = {};
  
  		this.decorate( 'checkChild' );
  		this.decorate( 'checkAttribute' );
  
  		this.on( 'checkAttribute', ( evt, args ) => {
  			args[ 0 ] = new SchemaContext( args[ 0 ] );
  		}, { priority: 'highest' } );
  
  		this.on( 'checkChild', ( evt, args ) => {
  			args[ 0 ] = new SchemaContext( args[ 0 ] );
  			args[ 1 ] = this.getDefinition( args[ 1 ] );
  		}, { priority: 'highest' } );
  	}
  
  	/**
  	 * Registers schema item. Can only be called once for every item name.
  	 *
  	 *		schema.register( 'paragraph', {
  	 *			inheritAllFrom: '$block'
  	 *		} );
  	 *
  	 * @param {String} itemName
  	 * @param {module:engine/model/schema~SchemaItemDefinition} definition
  	 */
  	register( itemName, definition ) {
  		if ( this._sourceDefinitions[ itemName ] ) {
  			/**
  			 * A single item cannot be registered twice in the schema.
  			 *
  			 * This situation may happen when:
  			 *
  			 * * Two or more plugins called {@link #register `register()`} with the same name. This will usually mean that
  			 * there is a collision between plugins which try to use the same element in the model. Unfortunately,
  			 * the only way to solve this is by modifying one of these plugins to use a unique model element name.
  			 * * A single plugin was loaded twice. This happens when it is installed by npm/yarn in two versions
  			 * and usually means one or more of the following issues:
  			 *     * a version mismatch (two of your dependencies require two different versions of this plugin),
  			 *     * incorrect imports (this plugin is somehow imported twice in a way which confuses webpack),
  			 *     * mess in `node_modules/` (`rm -rf node_modules/` may help).
  			 *
  			 * **Note:** Check the logged `itemName` to better understand which plugin was duplicated/conflicting.
  			 *
  			 * @param itemName The name of the model element that is being registered twice.
  			 * @error schema-cannot-register-item-twice
  			 */
  			throw new CKEditorError(
  				'schema-cannot-register-item-twice',
  				this,
  				{
  					itemName
  				}
  			);
  		}
  
  		this._sourceDefinitions[ itemName ] = [
  			Object.assign( {}, definition )
  		];
  
  		this._clearCache();
  	}
  
  	/**
  	 * Extends a {@link #register registered} item's definition.
  	 *
  	 * Extending properties such as `allowIn` will add more items to the existing properties,
  	 * while redefining properties such as `isBlock` will override the previously defined ones.
  	 *
  	 *		schema.register( 'foo', {
  	 *			allowIn: '$root',
  	 *			isBlock: true;
  	 *		} );
  	 *		schema.extend( 'foo', {
  	 *			allowIn: 'blockQuote',
  	 *			isBlock: false
  	 *		} );
  	 *
  	 *		schema.getDefinition( 'foo' );
  	 *		//	{
  	 *		//		allowIn: [ '$root', 'blockQuote' ],
  	 *		// 		isBlock: false
  	 *		//	}
  	 *
  	 * @param {String} itemName
  	 * @param {module:engine/model/schema~SchemaItemDefinition} definition
  	 */
  	extend( itemName, definition ) {
  		if ( !this._sourceDefinitions[ itemName ] ) {
  			/**
  			 * Cannot extend an item which was not registered yet.
  			 *
  			 * This error happens when a plugin tries to extend the schema definition of an item which was not
  			 * {@link #register registered} yet.
  			 *
  			 * @param itemName The name of the model element which is being extended.
  			 * @error schema-cannot-extend-missing-item
  			 */
  			throw new CKEditorError( 'schema-cannot-extend-missing-item', this, {
  				itemName
  			} );
  		}
  
  		this._sourceDefinitions[ itemName ].push( Object.assign( {}, definition ) );
  
  		this._clearCache();
  	}
  
  	/**
  	 * Returns data of all registered items.
  	 *
  	 * This method should normally be used for reflection purposes (e.g. defining a clone of a certain element,
  	 * checking a list of all block elements, etc).
  	 * Use specific methods (such as {@link #checkChild `checkChild()`} or {@link #isLimit `isLimit()`})
  	 * in other cases.
  	 *
  	 * @returns {Object.<String,module:engine/model/schema~SchemaCompiledItemDefinition>}
  	 */
  	getDefinitions() {
  		if ( !this._compiledDefinitions ) {
  			this._compile();
  		}
  
  		return this._compiledDefinitions;
  	}
  
  	/**
  	 * Returns a definition of the given item or `undefined` if an item is not registered.
  	 *
  	 * This method should normally be used for reflection purposes (e.g. defining a clone of a certain element,
  	 * checking a list of all block elements, etc).
  	 * Use specific methods (such as {@link #checkChild `checkChild()`} or {@link #isLimit `isLimit()`})
  	 * in other cases.
  	 *
  	 * @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
  	 * @returns {module:engine/model/schema~SchemaCompiledItemDefinition}
  	 */
  	getDefinition( item ) {
  		let itemName;
  
  		if ( typeof item == 'string' ) {
  			itemName = item;
  		} else if ( item.is && ( item.is( '$text' ) || item.is( '$textProxy' ) ) ) {
  			itemName = '$text';
  		}
  		// Element or module:engine/model/schema~SchemaContextItem.
  		else {
  			itemName = item.name;
  		}
  
  		return this.getDefinitions()[ itemName ];
  	}
  
  	/**
  	 * Returns `true` if the given item is registered in the schema.
  	 *
  	 *		schema.isRegistered( 'paragraph' ); // -> true
  	 *		schema.isRegistered( editor.model.document.getRoot() ); // -> true
  	 *		schema.isRegistered( 'foo' ); // -> false
  	 *
  	 * @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
  	 */
  	isRegistered( item ) {
  		return !!this.getDefinition( item );
  	}
  
  	/**
  	 * Returns `true` if the given item is defined to be
  	 * a block by the {@link module:engine/model/schema~SchemaItemDefinition}'s `isBlock` property.
  	 *
  	 *		schema.isBlock( 'paragraph' ); // -> true
  	 *		schema.isBlock( '$root' ); // -> false
  	 *
  	 *		const paragraphElement = writer.createElement( 'paragraph' );
  	 *		schema.isBlock( paragraphElement ); // -> true
  	 *
  	 * See the {@glink framework/guides/deep-dive/schema#block-elements Block elements} section of the Schema deep dive
  	 * guide for more details.
  	 *
  	 * @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
  	 */
  	isBlock( item ) {
  		const def = this.getDefinition( item );
  
  		return !!( def && def.isBlock );
  	}
  
  	/**
  	 * Returns `true` if the given item should be treated as a limit element.
  	 *
  	 * It considers an item to be a limit element if its
  	 * {@link module:engine/model/schema~SchemaItemDefinition}'s
  	 * {@link module:engine/model/schema~SchemaItemDefinition#isLimit `isLimit`} or
  	 * {@link module:engine/model/schema~SchemaItemDefinition#isObject `isObject`} property
  	 * was set to `true`.
  	 *
  	 *		schema.isLimit( 'paragraph' ); // -> false
  	 *		schema.isLimit( '$root' ); // -> true
  	 *		schema.isLimit( editor.model.document.getRoot() ); // -> true
  	 *		schema.isLimit( 'image' ); // -> true
  	 *
  	 * See the {@glink framework/guides/deep-dive/schema#limit-elements Limit elements} section of the Schema deep dive
  	 * guide for more details.
  	 *
  	 * @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
  	 */
  	isLimit( item ) {
  		const def = this.getDefinition( item );
  
  		if ( !def ) {
  			return false;
  		}
  
  		return !!( def.isLimit || def.isObject );
  	}
  
  	/**
  	 * Returns `true` if the given item should be treated as an object element.
  	 *
  	 * It considers an item to be an object element if its
  	 * {@link module:engine/model/schema~SchemaItemDefinition}'s
  	 * {@link module:engine/model/schema~SchemaItemDefinition#isObject `isObject`} property
  	 * was set to `true`.
  	 *
  	 *		schema.isObject( 'paragraph' ); // -> false
  	 *		schema.isObject( 'image' ); // -> true
  	 *
  	 *		const imageElement = writer.createElement( 'image' );
  	 *		schema.isObject( imageElement ); // -> true
  	 *
  	 * See the {@glink framework/guides/deep-dive/schema#object-elements Object elements} section of the Schema deep dive
  	 * guide for more details.
  	 *
  	 * @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
  	 */
  	isObject( item ) {
  		const def = this.getDefinition( item );
  
  		if ( !def ) {
  			return false;
  		}
  
  		// Note: Check out the implementation of #isLimit(), #isSelectable(), and #isContent()
  		// to understand why these three constitute an object.
  		return !!( def.isObject || ( def.isLimit && def.isSelectable && def.isContent ) );
  	}
  
  	/**
  	 * Returns `true` if the given item is defined to be
  	 * an inline element by the {@link module:engine/model/schema~SchemaItemDefinition}'s `isInline` property.
  	 *
  	 *		schema.isInline( 'paragraph' ); // -> false
  	 *		schema.isInline( 'softBreak' ); // -> true
  	 *
  	 *		const text = writer.createText( 'foo' );
  	 *		schema.isInline( text ); // -> true
  	 *
  	 * See the {@glink framework/guides/deep-dive/schema#inline-elements Inline elements} section of the Schema deep dive
  	 * guide for more details.
  	 *
  	 * @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
  	 */
  	isInline( item ) {
  		const def = this.getDefinition( item );
  
  		return !!( def && def.isInline );
  	}
  
  	/**
  	 * Returns `true` if the given item is defined to be
  	 * a selectable element by the {@link module:engine/model/schema~SchemaItemDefinition}'s `isSelectable` property.
  	 *
  	 *		schema.isSelectable( 'paragraph' ); // -> false
  	 *		schema.isSelectable( 'heading1' ); // -> false
  	 *		schema.isSelectable( 'image' ); // -> true
  	 *		schema.isSelectable( 'tableCell' ); // -> true
  	 *
  	 *		const text = writer.createText( 'foo' );
  	 *		schema.isSelectable( text ); // -> false
  	 *
  	 * See the {@glink framework/guides/deep-dive/schema#selectable-elements Selectable elements} section of the Schema deep dive}
  	 * guide for more details.
  	 *
  	 * @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
  	 */
  	isSelectable( item ) {
  		const def = this.getDefinition( item );
  
  		if ( !def ) {
  			return false;
  		}
  
  		return !!( def.isSelectable || def.isObject );
  	}
  
  	/**
  	 * Returns `true` if the given item is defined to be
  	 * a content by the {@link module:engine/model/schema~SchemaItemDefinition}'s `isContent` property.
  	 *
  	 *		schema.isContent( 'paragraph' ); // -> false
  	 *		schema.isContent( 'heading1' ); // -> false
  	 *		schema.isContent( 'image' ); // -> true
  	 *		schema.isContent( 'horizontalLine' ); // -> true
  	 *
  	 *		const text = writer.createText( 'foo' );
  	 *		schema.isContent( text ); // -> true
  	 *
  	 * See the {@glink framework/guides/deep-dive/schema#content-elements Content elements} section of the Schema deep dive}
  	 * guide for more details.
  	 *
  	 * @param {module:engine/model/item~Item|module:engine/model/schema~SchemaContextItem|String} item
  	 */
  	isContent( item ) {
  		const def = this.getDefinition( item );
  
  		if ( !def ) {
  			return false;
  		}
  
  		return !!( def.isContent || def.isObject );
  	}
  
  	/**
  	 * Checks whether the given node (`child`) can be a child of the given context.
  	 *
  	 *		schema.checkChild( model.document.getRoot(), paragraph ); // -> false
  	 *
  	 *		schema.register( 'paragraph', {
  	 *			allowIn: '$root'
  	 *		} );
  	 *		schema.checkChild( model.document.getRoot(), paragraph ); // -> true
  	 *
  	 * Note: When verifying whether the given node can be a child of the given context, the
  	 * schema also verifies the entire context &mdash; from its root to its last element. Therefore, it is possible
  	 * for `checkChild()` to return `false` even though the context's last element can contain the checked child.
  	 * It happens if one of the context's elements does not allow its child.
  	 *
  	 * @fires checkChild
  	 * @param {module:engine/model/schema~SchemaContextDefinition} context The context in which the child will be checked.
  	 * @param {module:engine/model/node~Node|String} def The child to check.
  	 */
  	checkChild( context, def ) {
  		// Note: context and child are already normalized here to a SchemaContext and SchemaCompiledItemDefinition.
  		if ( !def ) {
  			return false;
  		}
  
  		return this._checkContextMatch( def, context );
  	}
  
  	/**
  	 * Checks whether the given attribute can be applied in the given context (on the last
  	 * item of the context).
  	 *
  	 *		schema.checkAttribute( textNode, 'bold' ); // -> false
  	 *
  	 *		schema.extend( '$text', {
  	 *			allowAttributes: 'bold'
  	 *		} );
  	 *		schema.checkAttribute( textNode, 'bold' ); // -> true
  	 *
  	 * @fires checkAttribute
  	 * @param {module:engine/model/schema~SchemaContextDefinition} context The context in which the attribute will be checked.
  	 * @param {String} attributeName
  	 */
  	checkAttribute( context, attributeName ) {
  		const def = this.getDefinition( context.last );
  
  		if ( !def ) {
  			return false;
  		}
  
  		return def.allowAttributes.includes( attributeName );
  	}
  
  	/**
  	 * Checks whether the given element (`elementToMerge`) can be merged with the specified base element (`positionOrBaseElement`).
  	 *
  	 * In other words &mdash; whether `elementToMerge`'s children {@link #checkChild are allowed} in the `positionOrBaseElement`.
  	 *
  	 * This check ensures that elements merged with {@link module:engine/model/writer~Writer#merge `Writer#merge()`}
  	 * will be valid.
  	 *
  	 * Instead of elements, you can pass the instance of the {@link module:engine/model/position~Position} class as the
  	 * `positionOrBaseElement`. It means that the elements before and after the position will be checked whether they can be merged.
  	 *
  	 * @param {module:engine/model/position~Position|module:engine/model/element~Element} positionOrBaseElement The position or base
  	 * element to which the `elementToMerge` will be merged.
  	 * @param {module:engine/model/element~Element} elementToMerge The element to merge. Required if `positionOrBaseElement` is an element.
  	 * @returns {Boolean}
  	 */
  	checkMerge( positionOrBaseElement, elementToMerge = null ) {
  		if ( positionOrBaseElement instanceof Position ) {
  			const nodeBefore = positionOrBaseElement.nodeBefore;
  			const nodeAfter = positionOrBaseElement.nodeAfter;
  
  			if ( !( nodeBefore instanceof Element ) ) {
  				/**
  				 * The node before the merge position must be an element.
  				 *
  				 * @error schema-check-merge-no-element-before
  				 */
  				throw new CKEditorError(
  					'schema-check-merge-no-element-before',
  					this
  				);
  			}
  
  			if ( !( nodeAfter instanceof Element ) ) {
  				/**
  				 * The node after the merge position must be an element.
  				 *
  				 * @error schema-check-merge-no-element-after
  				 */
  				throw new CKEditorError(
  					'schema-check-merge-no-element-after',
  					this
  				);
  			}
  
  			return this.checkMerge( nodeBefore, nodeAfter );
  		}
  
  		for ( const child of elementToMerge.getChildren() ) {
  			if ( !this.checkChild( positionOrBaseElement, child ) ) {
  				return false;
  			}
  		}
  
  		return true;
  	}
  
  	/**
  	 * Allows registering a callback to the {@link #checkChild} method calls.
  	 *
  	 * Callbacks allow you to implement rules which are not otherwise possible to achieve
  	 * by using the declarative API of {@link module:engine/model/schema~SchemaItemDefinition}.
  	 * For example, by using this method you can disallow elements in specific contexts.
  	 *
  	 * This method is a shorthand for using the {@link #event:checkChild} event. For even better control,
  	 * you can use that event instead.
  	 *
  	 * Example:
  	 *
  	 *		// Disallow heading1 directly inside a blockQuote.
  	 *		schema.addChildCheck( ( context, childDefinition ) => {
  	 *			if ( context.endsWith( 'blockQuote' ) && childDefinition.name == 'heading1' ) {
  	 *				return false;
  	 *			}
  	 *		} );
  	 *
  	 * Which translates to:
  	 *
  	 *		schema.on( 'checkChild', ( evt, args ) => {
  	 *			const context = args[ 0 ];
  	 *			const childDefinition = args[ 1 ];
  	 *
  	 *			if ( context.endsWith( 'blockQuote' ) && childDefinition && childDefinition.name == 'heading1' ) {
  	 *				// Prevent next listeners from being called.
  	 *				evt.stop();
  	 *				// Set the checkChild()'s return value.
  	 *				evt.return = false;
  	 *			}
  	 *		}, { priority: 'high' } );
  	 *
  	 * @param {Function} callback The callback to be called. It is called with two parameters:
  	 * {@link module:engine/model/schema~SchemaContext} (context) instance and
  	 * {@link module:engine/model/schema~SchemaCompiledItemDefinition} (child-to-check definition).
  	 * The callback may return `true/false` to override `checkChild()`'s return value. If it does not return
  	 * a boolean value, the default algorithm (or other callbacks) will define `checkChild()`'s return value.
  	 */
  	addChildCheck( callback ) {
  		this.on( 'checkChild', ( evt, [ ctx, childDef ] ) => {
  			// checkChild() was called with a non-registered child.
  			// In 99% cases such check should return false, so not to overcomplicate all callbacks
  			// don't even execute them.
  			if ( !childDef ) {
  				return;
  			}
  
  			const retValue = callback( ctx, childDef );
  
  			if ( typeof retValue == 'boolean' ) {
  				evt.stop();
  				evt.return = retValue;
  			}
  		}, { priority: 'high' } );
  	}
  
  	/**
  	 * Allows registering a callback to the {@link #checkAttribute} method calls.
  	 *
  	 * Callbacks allow you to implement rules which are not otherwise possible to achieve
  	 * by using the declarative API of {@link module:engine/model/schema~SchemaItemDefinition}.
  	 * For example, by using this method you can disallow attribute if node to which it is applied
  	 * is contained within some other element (e.g. you want to disallow `bold` on `$text` within `heading1`).
  	 *
  	 * This method is a shorthand for using the {@link #event:checkAttribute} event. For even better control,
  	 * you can use that event instead.
  	 *
  	 * Example:
  	 *
  	 *		// Disallow bold on $text inside heading1.
  	 *		schema.addAttributeCheck( ( context, attributeName ) => {
  	 *			if ( context.endsWith( 'heading1 $text' ) && attributeName == 'bold' ) {
  	 *				return false;
  	 *			}
  	 *		} );
  	 *
  	 * Which translates to:
  	 *
  	 *		schema.on( 'checkAttribute', ( evt, args ) => {
  	 *			const context = args[ 0 ];
  	 *			const attributeName = args[ 1 ];
  	 *
  	 *			if ( context.endsWith( 'heading1 $text' ) && attributeName == 'bold' ) {
  	 *				// Prevent next listeners from being called.
  	 *				evt.stop();
  	 *				// Set the checkAttribute()'s return value.
  	 *				evt.return = false;
  	 *			}
  	 *		}, { priority: 'high' } );
  	 *
  	 * @param {Function} callback The callback to be called. It is called with two parameters:
  	 * {@link module:engine/model/schema~SchemaContext} (context) instance and attribute name.
  	 * The callback may return `true/false` to override `checkAttribute()`'s return value. If it does not return
  	 * a boolean value, the default algorithm (or other callbacks) will define `checkAttribute()`'s return value.
  	 */
  	addAttributeCheck( callback ) {
  		this.on( 'checkAttribute', ( evt, [ ctx, attributeName ] ) => {
  			const retValue = callback( ctx, attributeName );
  
  			if ( typeof retValue == 'boolean' ) {
  				evt.stop();
  				evt.return = retValue;
  			}
  		}, { priority: 'high' } );
  	}
  
  	/**
  	 * This method allows assigning additional metadata to the model attributes. For example,
  	 * {@link module:engine/model/schema~AttributeProperties `AttributeProperties#isFormatting` property} is
  	 * used to mark formatting attributes (like `bold` or `italic`).
  	 *
  	 *		// Mark bold as a formatting attribute.
  	 *		schema.setAttributeProperties( 'bold', {
  	 *			isFormatting: true
  	 *		} );
  	 *
  	 *		// Override code not to be considered a formatting markup.
  	 *		schema.setAttributeProperties( 'code', {
  	 *			isFormatting: false
  	 *		} );
  	 *
  	 * Properties are not limited to members defined in the
  	 * {@link module:engine/model/schema~AttributeProperties `AttributeProperties` type} and you can also use custom properties:
  	 *
  	 *		schema.setAttributeProperties( 'blockQuote', {
  	 *			customProperty: 'value'
  	 *		} );
  	 *
  	 * Subsequent calls with the same attribute will extend its custom properties:
  	 *
  	 *		schema.setAttributeProperties( 'blockQuote', {
  	 *			one: 1
  	 *		} );
  	 *
  	 *		schema.setAttributeProperties( 'blockQuote', {
  	 *			two: 2
  	 *		} );
  	 *
  	 *		console.log( schema.getAttributeProperties( 'blockQuote' ) );
  	 *		// Logs: { one: 1, two: 2 }
  	 *
  	 * @param {String} attributeName A name of the attribute to receive the properties.
  	 * @param {module:engine/model/schema~AttributeProperties} properties A dictionary of properties.
  	 */
  	setAttributeProperties( attributeName, properties ) {
  		this._attributeProperties[ attributeName ] = Object.assign( this.getAttributeProperties( attributeName ), properties );
  	}
  
  	/**
  	 * Returns properties associated with a given model attribute. See {@link #setAttributeProperties `setAttributeProperties()`}.
  	 *
  	 * @param {String} attributeName A name of the attribute.
  	 * @returns {module:engine/model/schema~AttributeProperties}
  	 */
  	getAttributeProperties( attributeName ) {
  		return this._attributeProperties[ attributeName ] || {};
  	}
  
  	/**
  	 * Returns the lowest {@link module:engine/model/schema~Schema#isLimit limit element} containing the entire
  	 * selection/range/position or the root otherwise.
  	 *
  	 * @param {module:engine/model/selection~Selection|module:engine/model/documentselection~DocumentSelection|
  	 * module:engine/model/range~Range|module:engine/model/position~Position} selectionOrRangeOrPosition
  	 * The selection/range/position to check.
  	 * @returns {module:engine/model/element~Element} The lowest limit element containing
  	 * the entire `selectionOrRangeOrPosition`.
  	 */
  	getLimitElement( selectionOrRangeOrPosition ) {
  		let element;
  
  		if ( selectionOrRangeOrPosition instanceof Position ) {
  			element = selectionOrRangeOrPosition.parent;
  		} else {
  			const ranges = selectionOrRangeOrPosition instanceof Range ?
  				[ selectionOrRangeOrPosition ] :
  				Array.from( selectionOrRangeOrPosition.getRanges() );
  
  			// Find the common ancestor for all selection's ranges.
  			element = ranges
  				.reduce( ( element, range ) => {
  					const rangeCommonAncestor = range.getCommonAncestor();
  
  					if ( !element ) {
  						return rangeCommonAncestor;
  					}
  
  					return element.getCommonAncestor( rangeCommonAncestor, { includeSelf: true } );
  				}, null );
  		}
  
  		while ( !this.isLimit( element ) ) {
  			if ( element.parent ) {
  				element = element.parent;
  			} else {
  				break;
  			}
  		}
  
  		return element;
  	}
  
  	/**
  	 * Checks whether the attribute is allowed in selection:
  	 *
  	 * * if the selection is not collapsed, then checks if the attribute is allowed on any of nodes in that range,
  	 * * if the selection is collapsed, then checks if on the selection position there's a text with the
  	 * specified attribute allowed.
  	 *
  	 * @param {module:engine/model/selection~Selection|module:engine/model/documentselection~DocumentSelection} selection
  	 * Selection which will be checked.
  	 * @param {String} attribute The name of the attribute to check.
  	 * @returns {Boolean}
  	 */
  	checkAttributeInSelection( selection, attribute ) {
  		if ( selection.isCollapsed ) {
  			const firstPosition = selection.getFirstPosition();
  			const context = [
  				...firstPosition.getAncestors(),
  				new Text( '', selection.getAttributes() )
  			];
  
  			// Check whether schema allows for a text with the attribute in the selection.
  			return this.checkAttribute( context, attribute );
  		} else {
  			const ranges = selection.getRanges();
  
  			// For all ranges, check nodes in them until you find a node that is allowed to have the attribute.
  			for ( const range of ranges ) {
  				for ( const value of range ) {
  					if ( this.checkAttribute( value.item, attribute ) ) {
  						// If we found a node that is allowed to have the attribute, return true.
  						return true;
  					}
  				}
  			}
  		}
  
  		// If we haven't found such node, return false.
  		return false;
  	}
  
  	/**
  	 * Transforms the given set of ranges into a set of ranges where the given attribute is allowed (and can be applied).
  	 *
  	 * @param {Array.<module:engine/model/range~Range>} ranges Ranges to be validated.
  	 * @param {String} attribute The name of the attribute to check.
  	 * @returns {Iterable.<module:engine/model/range~Range>} Ranges in which the attribute is allowed.
  	 */
  	* getValidRanges( ranges, attribute ) {
  		ranges = convertToMinimalFlatRanges( ranges );
  
  		for ( const range of ranges ) {
  			yield* this._getValidRangesForRange( range, attribute );
  		}
  	}
  
  	/**
  	 * Basing on given `position`, finds and returns a {@link module:engine/model/range~Range range} which is
  	 * nearest to that `position` and is a correct range for selection.
  	 *
  	 * The correct selection range might be collapsed when it is located in a position where the text node can be placed.
  	 * Non-collapsed range is returned when selection can be placed around element marked as an "object" in
  	 * the {@link module:engine/model/schema~Schema schema}.
  	 *
  	 * Direction of searching for the nearest correct selection range can be specified as:
  	 *
  	 * * `both` - searching will be performed in both ways,
  	 * * `forward` - searching will be performed only forward,
  	 * * `backward` - searching will be performed only backward.
  	 *
  	 * When valid selection range cannot be found, `null` is returned.
  	 *
  	 * @param {module:engine/model/position~Position} position Reference position where new selection range should be looked for.
  	 * @param {'both'|'forward'|'backward'} [direction='both'] Search direction.
  	 * @returns {module:engine/model/range~Range|null} Nearest selection range or `null` if one cannot be found.
  	 */
  	getNearestSelectionRange( position, direction = 'both' ) {
  		// Return collapsed range if provided position is valid.
  		if ( this.checkChild( position, '$text' ) ) {
  			return new Range( position );
  		}
  
  		let backwardWalker, forwardWalker;
  
  		// Never leave a limit element.
  		const limitElement = position.getAncestors().reverse().find( item => this.isLimit( item ) ) || position.root;
  
  		if ( direction == 'both' || direction == 'backward' ) {
  			backwardWalker = new TreeWalker( {
  				boundaries: Range._createIn( limitElement ),
  				startPosition: position,
  				direction: 'backward'
  			} );
  		}
  
  		if ( direction == 'both' || direction == 'forward' ) {
  			forwardWalker = new TreeWalker( {
  				boundaries: Range._createIn( limitElement ),
  				startPosition: position
  			} );
  		}
  
  		for ( const data of combineWalkers( backwardWalker, forwardWalker ) ) {
  			const type = ( data.walker == backwardWalker ? 'elementEnd' : 'elementStart' );
  			const value = data.value;
  
  			if ( value.type == type && this.isObject( value.item ) ) {
  				return Range._createOn( value.item );
  			}
  
  			if ( this.checkChild( value.nextPosition, '$text' ) ) {
  				return new Range( value.nextPosition );
  			}
  		}
  
  		return null;
  	}
  
  	/**
  	 * Tries to find position ancestors that allow to insert a given node.
  	 * It starts searching from the given position and goes node by node to the top of the model tree
  	 * as long as a {@link module:engine/model/schema~Schema#isLimit limit element}, an
  	 * {@link module:engine/model/schema~Schema#isObject object element} or a topmost ancestor is not reached.
  	 *
  	 * @param {module:engine/model/position~Position} position The position that the search will start from.
  	 * @param {module:engine/model/node~Node|String} node The node for which an allowed parent should be found or its name.
  	 * @returns {module:engine/model/element~Element|null} element Allowed parent or null if nothing was found.
  	 */
  	findAllowedParent( position, node ) {
  		let parent = position.parent;
  
  		while ( parent ) {
  			if ( this.checkChild( parent, node ) ) {
  				return parent;
  			}
  
  			// Do not split limit elements.
  			if ( this.isLimit( parent ) ) {
  				return null;
  			}
  
  			parent = parent.parent;
  		}
  
  		return null;
  	}
  
  	/**
  	 * Removes attributes disallowed by the schema.
  	 *
  	 * @param {Iterable.<module:engine/model/node~Node>} nodes Nodes that will be filtered.
  	 * @param {module:engine/model/writer~Writer} writer
  	 */
  	removeDisallowedAttributes( nodes, writer ) {
  		for ( const node of nodes ) {
  			// When node is a `Text` it has no children, so just filter it out.
  			if ( node.is( '$text' ) ) {
  				removeDisallowedAttributeFromNode( this, node, writer );
  			}
  			// In a case of `Element` iterates through positions between nodes inside this element
  			// and filter out node before the current position, or position parent when position
  			// is at start of an element. Using positions prevent from omitting merged nodes
  			// see https://github.com/ckeditor/ckeditor5-engine/issues/1789.
  			else {
  				const rangeInNode = Range._createIn( node );
  				const positionsInRange = rangeInNode.getPositions();
  
  				for ( const position of positionsInRange ) {
  					const item = position.nodeBefore || position.parent;
  
  					removeDisallowedAttributeFromNode( this, item, writer );
  				}
  			}
  		}
  	}
  
  	/**
  	 * Creates an instance of the schema context.
  	 *
  	 * @param {module:engine/model/schema~SchemaContextDefinition} context
  	 * @returns {module:engine/model/schema~SchemaContext}
  	 */
  	createContext( context ) {
  		return new SchemaContext( context );
  	}
  
  	/**
  	 * @private
  	 */
  	_clearCache() {
  		this._compiledDefinitions = null;
  	}
  
  	/**
  	 * @private
  	 */
  	_compile() {
  		const compiledDefinitions = {};
  		const sourceRules = this._sourceDefinitions;
  		const itemNames = Object.keys( sourceRules );
  
  		for ( const itemName of itemNames ) {
  			compiledDefinitions[ itemName ] = compileBaseItemRule( sourceRules[ itemName ], itemName );
  		}
  
  		for ( const itemName of itemNames ) {
  			compileAllowChildren( compiledDefinitions, itemName );
  		}
  
  		for ( const itemName of itemNames ) {
  			compileAllowContentOf( compiledDefinitions, itemName );
  		}
  
  		for ( const itemName of itemNames ) {
  			compileAllowWhere( compiledDefinitions, itemName );
  		}
  
  		for ( const itemName of itemNames ) {
  			compileAllowAttributesOf( compiledDefinitions, itemName );
  			compileInheritPropertiesFrom( compiledDefinitions, itemName );
  		}
  
  		for ( const itemName of itemNames ) {
  			cleanUpAllowIn( compiledDefinitions, itemName );
  			setupAllowChildren( compiledDefinitions, itemName );
  			cleanUpAllowAttributes( compiledDefinitions, itemName );
  		}
  
  		this._compiledDefinitions = compiledDefinitions;
  	}
  
  	/**
  	 * @private
  	 * @param {module:engine/model/schema~SchemaCompiledItemDefinition} def
  	 * @param {module:engine/model/schema~SchemaContext} context
  	 * @param {Number} contextItemIndex
  	 */
  	_checkContextMatch( def, context, contextItemIndex = context.length - 1 ) {
  		const contextItem = context.getItem( contextItemIndex );
  
  		if ( def.allowIn.includes( contextItem.name ) ) {
  			if ( contextItemIndex == 0 ) {
  				return true;
  			} else {
  				const parentRule = this.getDefinition( contextItem );
  
  				return this._checkContextMatch( parentRule, context, contextItemIndex - 1 );
  			}
  		} else {
  			return false;
  		}
  	}
  
  	/**
  	 * Takes a flat range and an attribute name. Traverses the range recursively and deeply to find and return all ranges
  	 * inside the given range on which the attribute can be applied.
  	 *
  	 * This is a helper function for {@link ~Schema#getValidRanges}.
  	 *
  	 * @private
  	 * @param {module:engine/model/range~Range} range The range to process.
  	 * @param {String} attribute The name of the attribute to check.
  	 * @returns {Iterable.<module:engine/model/range~Range>} Ranges in which the attribute is allowed.
  	 */
  	* _getValidRangesForRange( range, attribute ) {
  		let start = range.start;
  		let end = range.start;
  
  		for ( const item of range.getItems( { shallow: true } ) ) {
  			if ( item.is( 'element' ) ) {
  				yield* this._getValidRangesForRange( Range._createIn( item ), attribute );
  			}
  
  			if ( !this.checkAttribute( item, attribute ) ) {
  				if ( !start.isEqual( end ) ) {
  					yield new Range( start, end );
  				}
  
  				start = Position._createAfter( item );
  			}
  
  			end = Position._createAfter( item );
  		}
  
  		if ( !start.isEqual( end ) ) {
  			yield new Range( start, end );
  		}
  	}
  }
  
  mix( Schema, ObservableMixin );
  
  /**
   * Event fired when the {@link #checkChild} method is called. It allows plugging in
   * additional behavior, for example implementing rules which cannot be defined using the declarative
   * {@link module:engine/model/schema~SchemaItemDefinition} interface.
   *
   * **Note:** The {@link #addChildCheck} method is a more handy way to register callbacks. Internally,
   * it registers a listener to this event but comes with a simpler API and it is the recommended choice
   * in most of the cases.
   *
   * The {@link #checkChild} method fires an event because it is
   * {@link module:utils/observablemixin~ObservableMixin#decorate decorated} with it. Thanks to that you can
   * use this event in various ways, but the most important use case is overriding standard behavior of the
   * `checkChild()` method. Let's see a typical listener template:
   *
   *		schema.on( 'checkChild', ( evt, args ) => {
   *			const context = args[ 0 ];
   *			const childDefinition = args[ 1 ];
   *		}, { priority: 'high' } );
   *
   * The listener is added with a `high` priority to be executed before the default method is really called. The `args` callback
   * parameter contains arguments passed to `checkChild( context, child )`. However, the `context` parameter is already
   * normalized to a {@link module:engine/model/schema~SchemaContext} instance and `child` to a
   * {@link module:engine/model/schema~SchemaCompiledItemDefinition} instance, so you do not have to worry about
   * the various ways how `context` and `child` may be passed to `checkChild()`.
   *
   * **Note:** `childDefinition` may be `undefined` if `checkChild()` was called with a non-registered element.
   *
   * So, in order to implement a rule "disallow `heading1` in `blockQuote`", you can add such a listener:
   *
   *		schema.on( 'checkChild', ( evt, args ) => {
   *			const context = args[ 0 ];
   *			const childDefinition = args[ 1 ];
   *
   *			if ( context.endsWith( 'blockQuote' ) && childDefinition && childDefinition.name == 'heading1' ) {
   *				// Prevent next listeners from being called.
   *				evt.stop();
   *				// Set the checkChild()'s return value.
   *				evt.return = false;
   *			}
   *		}, { priority: 'high' } );
   *
   * Allowing elements in specific contexts will be a far less common use case, because it is normally handled by the
   * `allowIn` rule from {@link module:engine/model/schema~SchemaItemDefinition}. But if you have a complex scenario
   * where `listItem` should be allowed only in element `foo` which must be in element `bar`, then this would be the way:
   *
   *		schema.on( 'checkChild', ( evt, args ) => {
   *			const context = args[ 0 ];
   *			const childDefinition = args[ 1 ];
   *
   *			if ( context.endsWith( 'bar foo' ) && childDefinition.name == 'listItem' ) {
   *				// Prevent next listeners from being called.
   *				evt.stop();
   *				// Set the checkChild()'s return value.
   *				evt.return = true;
   *			}
   *		}, { priority: 'high' } );
   *
   * @event checkChild
   * @param {Array} args The `checkChild()`'s arguments.
   */
  
  /**
   * Event fired when the {@link #checkAttribute} method is called. It allows plugging in
   * additional behavior, for example implementing rules which cannot be defined using the declarative
   * {@link module:engine/model/schema~SchemaItemDefinition} interface.
   *
   * **Note:** The {@link #addAttributeCheck} method is a more handy way to register callbacks. Internally,
   * it registers a listener to this event but comes with a simpler API and it is the recommended choice
   * in most of the cases.
   *
   * The {@link #checkAttribute} method fires an event because it is
   * {@link module:utils/observablemixin~ObservableMixin#decorate decorated} with it. Thanks to that you can
   * use this event in various ways, but the most important use case is overriding the standard behavior of the
   * `checkAttribute()` method. Let's see a typical listener template:
   *
   *		schema.on( 'checkAttribute', ( evt, args ) => {
   *			const context = args[ 0 ];
   *			const attributeName = args[ 1 ];
   *		}, { priority: 'high' } );
   *
   * The listener is added with a `high` priority to be executed before the default method is really called. The `args` callback
   * parameter contains arguments passed to `checkAttribute( context, attributeName )`. However, the `context` parameter is already
   * normalized to a {@link module:engine/model/schema~SchemaContext} instance, so you do not have to worry about
   * the various ways how `context` may be passed to `checkAttribute()`.
   *
   * So, in order to implement a rule "disallow `bold` in a text which is in a `heading1`, you can add such a listener:
   *
   *		schema.on( 'checkAttribute', ( evt, args ) => {
   *			const context = args[ 0 ];
   *			const attributeName = args[ 1 ];
   *
   *			if ( context.endsWith( 'heading1 $text' ) && attributeName == 'bold' ) {
   *				// Prevent next listeners from being called.
   *				evt.stop();
   *				// Set the checkAttribute()'s return value.
   *				evt.return = false;
   *			}
   *		}, { priority: 'high' } );
   *
   * Allowing attributes in specific contexts will be a far less common use case, because it is normally handled by the
   * `allowAttributes` rule from {@link module:engine/model/schema~SchemaItemDefinition}. But if you have a complex scenario
   * where `bold` should be allowed only in element `foo` which must be in element `bar`, then this would be the way:
   *
   *		schema.on( 'checkAttribute', ( evt, args ) => {
   *			const context = args[ 0 ];
   *			const attributeName = args[ 1 ];
   *
   *			if ( context.endsWith( 'bar foo $text' ) && attributeName == 'bold' ) {
   *				// Prevent next listeners from being called.
   *				evt.stop();
   *				// Set the checkAttribute()'s return value.
   *				evt.return = true;
   *			}
   *		}, { priority: 'high' } );
   *
   * @event checkAttribute
   * @param {Array} args The `checkAttribute()`'s arguments.
   */
  
  /**
   * A definition of a {@link module:engine/model/schema~Schema schema} item.
   *
   * You can define the following rules:
   *
   * * {@link ~SchemaItemDefinition#allowIn `allowIn`} &ndash; Defines in which other items this item will be allowed.
   * * {@link ~SchemaItemDefinition#allowChildren `allowChildren`} &ndash; Defines which other items are allowed inside this item.
   * * {@link ~SchemaItemDefinition#allowAttributes `allowAttributes`} &ndash; Defines allowed attributes of the given item.
   * * {@link ~SchemaItemDefinition#allowContentOf `allowContentOf`} &ndash; Inherits "allowed children" from other items.
   * * {@link ~SchemaItemDefinition#allowWhere `allowWhere`} &ndash; Inherits "allowed in" from other items.
   * * {@link ~SchemaItemDefinition#allowAttributesOf `allowAttributesOf`} &ndash; Inherits attributes from other items.
   * * {@link ~SchemaItemDefinition#inheritTypesFrom `inheritTypesFrom`} &ndash; Inherits `is*` properties of other items.
   * * {@link ~SchemaItemDefinition#inheritAllFrom `inheritAllFrom`} &ndash;
   * A shorthand for `allowContentOf`, `allowWhere`, `allowAttributesOf`, `inheritTypesFrom`.
   *
   * # The `is*` properties
   *
   * There are a couple commonly used `is*` properties. Their role is to assign additional semantics to schema items.
   * You can define more properties but you will also need to implement support for them in the existing editor features.
   *
   * * {@link ~SchemaItemDefinition#isBlock `isBlock`} &ndash; Whether this item is paragraph-like.
   * Generally speaking, content is usually made out of blocks like paragraphs, list items, images, headings, etc.
   * * {@link ~SchemaItemDefinition#isInline `isInline`} &ndash; Whether an item is "text-like" and should be treated as an inline node.
   * Examples of inline elements: `$text`, `softBreak` (`<br>`), etc.
   * * {@link ~SchemaItemDefinition#isLimit `isLimit`} &ndash; It can be understood as whether this element
   * should not be split by <kbd>Enter</kbd>. Examples of limit elements: `$root`, table cell, image caption, etc.
   * In other words, all actions that happen inside a limit element are limited to its content.
   * All objects are treated as limit elements, too.
   * * {@link ~SchemaItemDefinition#isObject `isObject`} &ndash; Whether an item is "self-contained" and should be treated as a whole.
   * Examples of object elements: `image`, `table`, `video`, etc. An object is also a limit, so
   * {@link module:engine/model/schema~Schema#isLimit `isLimit()`} returns `true` for object elements automatically.
   *
   * Read more about the meaning of these types in the
   * {@glink framework/guides/deep-dive/schema#defining-additional-semantics dedicated section of the Schema deep dive} guide.
   *
   * # Generic items
   *
   * There are three basic generic items: `$root`, `$block` and `$text`.
   * They are defined as follows:
   *
   *		this.schema.register( '$root', {
   *			isLimit: true
   *		} );
   *		this.schema.register( '$block', {
   *			allowIn: '$root',
   *			isBlock: true
   *		} );
   *		this.schema.register( '$text', {
   *			allowIn: '$block',
   *			isInline: true
   *		} );
   *
   * They reflect typical editor content that is contained within one root, consists of several blocks
   * (paragraphs, lists items, headings, images) which, in turn, may contain text inside.
   *
   * By inheriting from the generic items you can define new items which will get extended by other editor features.
   * Read more about generic types in the {@glink framework/guides/deep-dive/schema Schema deep dive} guide.
   *
   * # Example definitions
   *
   * Allow `paragraph` in roots and block quotes:
   *
   *		schema.register( 'paragraph', {
   *			allowIn: [ '$root', 'blockQuote' ],
   *			isBlock: true
   *		} );
   *
   * Allow `paragraph` everywhere where `$block` is allowed (i.e. in `$root`):
   *
   *		schema.register( 'paragraph', {
   *			allowWhere: '$block',
   *			isBlock: true
   *		} );
   *
   * Allow `paragraph` inside a `$root` and allow `$text` as a `paragraph` child:
   *
   *		schema.register( 'paragraph', {
   *			allowIn: '$root',
   *			allowChildren: '$text',
   *			isBlock: true
   *		} );
   *
   * Make `image` a block object, which is allowed everywhere where `$block` is.
   * Also, allow `src` and `alt` attributes in it:
   *
   *		schema.register( 'image', {
   *			allowWhere: '$block',
   *			allowAttributes: [ 'src', 'alt' ],
   *			isBlock: true,
   *			isObject: true
   *		} );
   *
   * Make `caption` allowed in `image` and make it allow all the content of `$block`s (usually, `$text`).
   * Also, mark it as a limit element so it cannot be split:
   *
   *		schema.register( 'caption', {
   *			allowIn: 'image',
   *			allowContentOf: '$block',
   *			isLimit: true
   *		} );
   *
   * Make `listItem` inherit all from `$block` but also allow additional attributes:
   *
   *		schema.register( 'listItem', {
   *			inheritAllFrom: '$block',
   *			allowAttributes: [ 'listType', 'listIndent' ]
   *		} );
   *
   * Which translates to:
   *
   *		schema.register( 'listItem', {
   *			allowWhere: '$block',
   *			allowContentOf: '$block',
   *			allowAttributesOf: '$block',
   *			inheritTypesFrom: '$block',
   *			allowAttributes: [ 'listType', 'listIndent' ]
   *		} );
   *
   * # Tips
   *
   * * Check schema definitions of existing features to see how they are defined.
   * * If you want to publish your feature so other developers can use it, try to use
   * generic items as much as possible.
   * * Keep your model clean. Limit it to the actual data and store information in a normalized way.
   * * Remember about defining the `is*` properties. They do not affect the allowed structures, but they can
   * affect how the editor features treat your elements.
   *
   * @typedef {Object} module:engine/model/schema~SchemaItemDefinition
   *
   * @property {String|Array.<String>} allowIn Defines in which other items this item will be allowed.
   * @property {String|Array.<String>} allowChildren Defines which other items are allowed inside this item.
   * @property {String|Array.<String>} allowAttributes Defines allowed attributes of the given item.
   * @property {String|Array.<String>} allowContentOf Inherits "allowed children" from other items.
   * @property {String|Array.<String>} allowWhere Inherits "allowed in" from other items.
   * @property {String|Array.<String>} allowAttributesOf Inherits attributes from other items.
   * @property {String|Array.<String>} inheritTypesFrom Inherits `is*` properties of other items.
   * @property {String} inheritAllFrom A shorthand for `allowContentOf`, `allowWhere`, `allowAttributesOf`, `inheritTypesFrom`.
   *
   * @property {Boolean} isBlock
   * Whether this item is paragraph-like. Generally speaking, content is usually made out of blocks
   * like paragraphs, list items, images, headings, etc. All these elements are marked as blocks. A block
   * should not allow another block inside. Note: There is also the `$block` generic item which has `isBlock` set to `true`.
   * Most block type items will inherit from `$block` (through `inheritAllFrom`).
   *
   * Read more about the block elements in the
   * {@glink framework/guides/deep-dive/schema#block-elements Block elements} section of the Schema deep dive} guide.
   *
   * @property {Boolean} isInline
   * Whether an item is "text-like" and should be treated as an inline node. Examples of inline elements:
   * `$text`, `softBreak` (`<br>`), etc.
   *
   * Read more about the inline elements in the
   * {@glink framework/guides/deep-dive/schema#inline-elements Inline elements} section of the Schema deep dive} guide.
   *
   * @property {Boolean} isLimit
   * It can be understood as whether this element should not be split by <kbd>Enter</kbd>.
   * Examples of limit elements: `$root`, table cell, image caption, etc. In other words, all actions that happen inside
   * a limit element are limited to its content.
   *
   * Read more about the limit elements in the
   * {@glink framework/guides/deep-dive/schema#limit-elements Limit elements} section of the Schema deep dive} guide.
   *
   * @property {Boolean} isObject
   * Whether an item is "self-contained" and should be treated as a whole. Examples of object elements:
   * `image`, `table`, `video`, etc.
   *
   * **Note:** An object is also a limit, so
   * {@link module:engine/model/schema~Schema#isLimit `isLimit()`} returns `true` for object elements automatically.
   *
   * Read more about the object elements in the
   * {@glink framework/guides/deep-dive/schema#object-elements Object elements} section of the Schema deep dive} guide.
   *
   * @property {Boolean} isSelectable
   * `true` when an element should be selectable as a whole by the user. Examples of selectable elements: `image`, `table`, `tableCell`, etc.
   *
   * **Note:** An object is also a selectable element, so
   * {@link module:engine/model/schema~Schema#isSelectable `isSelectable()`} returns `true` for object elements automatically.
   *
   * Read more about selectable elements in the
   * {@glink framework/guides/deep-dive/schema#selectable-elements Selectable elements} section of the Schema deep dive} guide.
   *
   * @property {Boolean} isContent
   * An item is a content when it always finds its way to the editor data output regardless of the number and type of its descendants.
   * Examples of content elements: `$text`, `image`, `table`, etc. (but not `paragraph`, `heading1` or `tableCell`).
   *
   * **Note:** An object is also a content element, so
   * {@link module:engine/model/schema~Schema#isContent `isContent()`} returns `true` for object elements automatically.
   *
   * Read more about content elements in the
   * {@glink framework/guides/deep-dive/schema#content-elements Content elements} section of the Schema deep dive} guide.
   */
  
  /**
   * A simplified version of {@link module:engine/model/schema~SchemaItemDefinition} after
   * compilation by the {@link module:engine/model/schema~Schema schema}.
   * Rules fed to the schema by {@link module:engine/model/schema~Schema#register}
   * and {@link module:engine/model/schema~Schema#extend} methods are defined in the
   * {@link module:engine/model/schema~SchemaItemDefinition} format.
   * Later on, they are compiled to `SchemaCompiledItemDefinition` so when you use e.g.
   * the {@link module:engine/model/schema~Schema#getDefinition} method you get the compiled version.
   *
   * The compiled version contains only the following properties:
   *
   * * The `name` property,
   * * The `is*` properties,
   * * The `allowIn` array,
   * * The `allowChildren` array,
   * * The `allowAttributes` array.
   *
   * @typedef {Object} module:engine/model/schema~SchemaCompiledItemDefinition
   */
  
  /**
   * A schema context &mdash; a list of ancestors of a given position in the document.
   *
   * Considering such position:
   *
   *		<$root>
   *			<blockQuote>
   *				<paragraph>
   *					^
   *				</paragraph>
   *			</blockQuote>
   *		</$root>
   *
   * The context of this position is its {@link module:engine/model/position~Position#getAncestors lists of ancestors}:
   *
   *		[ rootElement, blockQuoteElement, paragraphElement ]
   *
   * Contexts are used in the {@link module:engine/model/schema~Schema#event:checkChild `Schema#checkChild`} and
   * {@link module:engine/model/schema~Schema#event:checkAttribute `Schema#checkAttribute`} events as a definition
   * of a place in the document where the check occurs. The context instances are created based on the first arguments
   * of the {@link module:engine/model/schema~Schema#checkChild `Schema#checkChild()`} and
   * {@link module:engine/model/schema~Schema#checkAttribute `Schema#checkAttribute()`} methods so when
   * using these methods you need to use {@link module:engine/model/schema~SchemaContextDefinition}s.
   */
  export class SchemaContext {
  	/**
  	 * Creates an instance of the context.
  	 *
  	 * @param {module:engine/model/schema~SchemaContextDefinition} context
  	 */
  	constructor( context ) {
  		if ( context instanceof SchemaContext ) {
  			return context;
  		}
  
  		if ( typeof context == 'string' ) {
  			context = [ context ];
  		} else if ( !Array.isArray( context ) ) {
  			// `context` is item or position.
  			// Position#getAncestors() doesn't accept any parameters but it works just fine here.
  			context = context.getAncestors( { includeSelf: true } );
  		}
  
  		this._items = context.map( mapContextItem );
  	}
  
  	/**
  	 * The number of items.
  	 *
  	 * @type {Number}
  	 */
  	get length() {
  		return this._items.length;
  	}
  
  	/**
  	 * The last item (the lowest node).
  	 *
  	 * @type {module:engine/model/schema~SchemaContextItem}
  	 */
  	get last() {
  		return this._items[ this._items.length - 1 ];
  	}
  
  	/**
  	 * Iterable interface.
  	 *
  	 * Iterates over all context items.
  	 *
  	 * @returns {Iterable.<module:engine/model/schema~SchemaContextItem>}
  	 */
  	[ Symbol.iterator ]() {
  		return this._items[ Symbol.iterator ]();
  	}
  
  	/**
  	 * Returns a new schema context instance with an additional item.
  	 *
  	 * Item can be added as:
  	 *
  	 * 		const context = new SchemaContext( [ '$root' ] );
  	 *
  	 * 		// An element.
  	 * 		const fooElement = writer.createElement( 'fooElement' );
  	 * 		const newContext = context.push( fooElement ); // [ '$root', 'fooElement' ]
  	 *
  	 * 		// A text node.
  	 * 		const text = writer.createText( 'foobar' );
  	 * 		const newContext = context.push( text ); // [ '$root', '$text' ]
  	 *
  	 * 		// A string (element name).
  	 * 		const newContext = context.push( 'barElement' ); // [ '$root', 'barElement' ]
  	 *
  	 * **Note** {@link module:engine/model/node~Node} that is already in the model tree will be added as the only item
  	 * (without ancestors).
  	 *
  	 * @param {String|module:engine/model/node~Node|Array<String|module:engine/model/node~Node>} item An item that will be added
  	 * to the current context.
  	 * @returns {module:engine/model/schema~SchemaContext} A new schema context instance with an additional item.
  	 */
  	push( item ) {
  		const ctx = new SchemaContext( [ item ] );
  
  		ctx._items = [ ...this._items, ...ctx._items ];
  
  		return ctx;
  	}
  
  	/**
  	 * Gets an item on the given index.
  	 *
  	 * @returns {module:engine/model/schema~SchemaContextItem}
  	 */
  	getItem( index ) {
  		return this._items[ index ];
  	}
  
  	/**
  	 * Returns the names of items.
  	 *
  	 * @returns {Iterable.<String>}
  	 */
  	* getNames() {
  		yield* this._items.map( item => item.name );
  	}
  
  	/**
  	 * Checks whether the context ends with the given nodes.
  	 *
  	 *		const ctx = new SchemaContext( [ rootElement, paragraphElement, textNode ] );
  	 *
  	 *		ctx.endsWith( '$text' ); // -> true
  	 *		ctx.endsWith( 'paragraph $text' ); // -> true
  	 *		ctx.endsWith( '$root' ); // -> false
  	 *		ctx.endsWith( 'paragraph' ); // -> false
  	 *
  	 * @param {String} query
  	 * @returns {Boolean}
  	 */
  	endsWith( query ) {
  		return Array.from( this.getNames() ).join( ' ' ).endsWith( query );
  	}
  
  	/**
  	 * Checks whether the context starts with the given nodes.
  	 *
  	 *		const ctx = new SchemaContext( [ rootElement, paragraphElement, textNode ] );
  	 *
  	 *		ctx.endsWith( '$root' ); // -> true
  	 *		ctx.endsWith( '$root paragraph' ); // -> true
  	 *		ctx.endsWith( '$text' ); // -> false
  	 *		ctx.endsWith( 'paragraph' ); // -> false
  	 *
  	 * @param {String} query
  	 * @returns {Boolean}
  	 */
  	startsWith( query ) {
  		return Array.from( this.getNames() ).join( ' ' ).startsWith( query );
  	}
  }
  
  /**
   * The definition of a {@link module:engine/model/schema~SchemaContext schema context}.
   *
   * Contexts can be created in multiple ways:
   *
   * * By defining a **node** – in this cases this node and all its ancestors will be used.
   * * By defining a **position** in the document – in this case all its ancestors will be used.
   * * By defining an **array of nodes** – in this case this array defines the entire context.
   * * By defining a **name of node** - in this case node will be "mocked". It is not recommended because context
   * will be unrealistic (e.g. attributes of these nodes are not specified). However, at times this may be the only
   * way to define the context (e.g. when checking some hypothetical situation).
   * * By defining an **array of node names** (potentially, mixed with real nodes) – The same as **name of node**
   * but it is possible to create a path.
   * * By defining a {@link module:engine/model/schema~SchemaContext} instance - in this case the same instance as provided
   * will be return.
   *
   * Examples of context definitions passed to the {@link module:engine/model/schema~Schema#checkChild `Schema#checkChild()`}
   * method:
   *
   *		// Assuming that we have a $root > blockQuote > paragraph structure, the following code
   *		// will check node 'foo' in the following context:
   *		// [ rootElement, blockQuoteElement, paragraphElement ]
   *		const contextDefinition = paragraphElement;
   * 		const childToCheck = 'foo';
   *		schema.checkChild( contextDefinition, childToCheck );
   *
   *		// Also check in [ rootElement, blockQuoteElement, paragraphElement ].
   *		schema.checkChild( model.createPositionAt( paragraphElement, 0 ), 'foo' );
   *
   *		// Check in [ rootElement, paragraphElement ].
   *		schema.checkChild( [ rootElement, paragraphElement ], 'foo' );
   *
   *		// Check only fakeParagraphElement.
   *		schema.checkChild( 'paragraph', 'foo' );
   *
   *		// Check in [ fakeRootElement, fakeBarElement, paragraphElement ].
   *		schema.checkChild( [ '$root', 'bar', paragraphElement ], 'foo' );
   *
   * All these `checkChild()` calls will fire {@link module:engine/model/schema~Schema#event:checkChild `Schema#checkChild`}
   * events in which `args[ 0 ]` is an instance of the context. Therefore, you can write a listener like this:
   *
   *		schema.on( 'checkChild', ( evt, args ) => {
   *			const ctx = args[ 0 ];
   *
   *			console.log( Array.from( ctx.getNames() ) );
   *		} );
   *
   * Which will log the following:
   *
   *		[ '$root', 'blockQuote', 'paragraph' ]
   *		[ '$root', 'paragraph' ]
   *		[ '$root', 'bar', 'paragraph' ]
   *
   * Note: When using the {@link module:engine/model/schema~Schema#checkAttribute `Schema#checkAttribute()`} method
   * you may want to check whether a text node may have an attribute. A {@link module:engine/model/text~Text} is a
   * correct way to define a context so you can do this:
   *
   *		schema.checkAttribute( textNode, 'bold' );
   *
   * But sometimes you want to check whether a text at a given position might've had some attribute,
   * in which case you can create a context by mixing in an array of elements with a `'$text'` string:
   *
   *		// Check in [ rootElement, paragraphElement, textNode ].
   *		schema.checkChild( [ ...positionInParagraph.getAncestors(), '$text' ], 'bold' );
   *
   * @typedef {module:engine/model/node~Node|module:engine/model/position~Position|module:engine/model/schema~SchemaContext|
   * String|Array.<String|module:engine/model/node~Node>} module:engine/model/schema~SchemaContextDefinition
   */
  
  /**
   * An item of the {@link module:engine/model/schema~SchemaContext schema context}.
   *
   * It contains 3 properties:
   *
   * * `name` – the name of this item,
   * * `* getAttributeKeys()` – a generator of keys of item attributes,
   * * `getAttribute( keyName )` – a method to get attribute values.
   *
   * The context item interface is a highly simplified version of {@link module:engine/model/node~Node} and its role
   * is to expose only the information which schema checks are able to provide (which is the name of the node and
   * node's attributes).
   *
   *		schema.on( 'checkChild', ( evt, args ) => {
   *			const ctx = args[ 0 ];
   *			const firstItem = ctx.getItem( 0 );
   *
   *			console.log( firstItem.name ); // -> '$root'
   *			console.log( firstItem.getAttribute( 'foo' ) ); // -> 'bar'
   *			console.log( Array.from( firstItem.getAttributeKeys() ) ); // -> [ 'foo', 'faa' ]
   *		} );
   *
   * @typedef {Object} module:engine/model/schema~SchemaContextItem
   */
  
  /**
   * A structure containing additional metadata describing the attribute.
   *
   * See {@link module:engine/model/schema~Schema#setAttributeProperties `Schema#setAttributeProperties()`} for usage examples.
   *
   * @typedef {Object} module:engine/model/schema~AttributeProperties
   * @property {Boolean} [isFormatting] Indicates that the attribute should be considered as a visual formatting, like `bold`, `italic` or
   * `fontSize` rather than semantic attribute (such as `src`, `listType`, etc.). For example, it is used by the "Remove format" feature.
   * @property {Boolean} [copyOnEnter] Indicates that given text attribute should be copied to the next block when enter is pressed.
   */
  
  function compileBaseItemRule( sourceItemRules, itemName ) {
  	const itemRule = {
  		name: itemName,
  
  		allowIn: [],
  		allowContentOf: [],
  		allowWhere: [],
  
  		allowAttributes: [],
  		allowAttributesOf: [],
  
  		allowChildren: [],
  
  		inheritTypesFrom: []
  	};
  
  	copyTypes( sourceItemRules, itemRule );
  
  	copyProperty( sourceItemRules, itemRule, 'allowIn' );
  	copyProperty( sourceItemRules, itemRule, 'allowContentOf' );
  	copyProperty( sourceItemRules, itemRule, 'allowWhere' );
  
  	copyProperty( sourceItemRules, itemRule, 'allowAttributes' );
  	copyProperty( sourceItemRules, itemRule, 'allowAttributesOf' );
  
  	copyProperty( sourceItemRules, itemRule, 'allowChildren' );
  
  	copyProperty( sourceItemRules, itemRule, 'inheritTypesFrom' );
  
  	makeInheritAllWork( sourceItemRules, itemRule );
  
  	return itemRule;
  }
  
  function compileAllowChildren( compiledDefinitions, itemName ) {
  	const item = compiledDefinitions[ itemName ];
  
  	for ( const allowChildrenItem of item.allowChildren ) {
  		const allowedChildren = compiledDefinitions[ allowChildrenItem ];
  
  		// The allowChildren property may point to an unregistered element.
  		if ( !allowedChildren ) {
  			continue;
  		}
  
  		allowedChildren.allowIn.push( itemName );
  	}
  
  	// The allowIn property already includes correct items, reset the allowChildren property
  	// to avoid duplicates later when setting up compilation results.
  	item.allowChildren.length = 0;
  }
  
  function compileAllowContentOf( compiledDefinitions, itemName ) {
  	for ( const allowContentOfItemName of compiledDefinitions[ itemName ].allowContentOf ) {
  		// The allowContentOf property may point to an unregistered element.
  		if ( compiledDefinitions[ allowContentOfItemName ] ) {
  			const allowedChildren = getAllowedChildren( compiledDefinitions, allowContentOfItemName );
  
  			allowedChildren.forEach( allowedItem => {
  				allowedItem.allowIn.push( itemName );
  			} );
  		}
  	}
  
  	delete compiledDefinitions[ itemName ].allowContentOf;
  }
  
  function compileAllowWhere( compiledDefinitions, itemName ) {
  	for ( const allowWhereItemName of compiledDefinitions[ itemName ].allowWhere ) {
  		const inheritFrom = compiledDefinitions[ allowWhereItemName ];
  
  		// The allowWhere property may point to an unregistered element.
  		if ( inheritFrom ) {
  			const allowedIn = inheritFrom.allowIn;
  
  			compiledDefinitions[ itemName ].allowIn.push( ...allowedIn );
  		}
  	}
  
  	delete compiledDefinitions[ itemName ].allowWhere;
  }
  
  function compileAllowAttributesOf( compiledDefinitions, itemName ) {
  	for ( const allowAttributeOfItem of compiledDefinitions[ itemName ].allowAttributesOf ) {
  		const inheritFrom = compiledDefinitions[ allowAttributeOfItem ];
  
  		if ( inheritFrom ) {
  			const inheritAttributes = inheritFrom.allowAttributes;
  
  			compiledDefinitions[ itemName ].allowAttributes.push( ...inheritAttributes );
  		}
  	}
  
  	delete compiledDefinitions[ itemName ].allowAttributesOf;
  }
  
  function compileInheritPropertiesFrom( compiledDefinitions, itemName ) {
  	const item = compiledDefinitions[ itemName ];
  
  	for ( const inheritPropertiesOfItem of item.inheritTypesFrom ) {
  		const inheritFrom = compiledDefinitions[ inheritPropertiesOfItem ];
  
  		if ( inheritFrom ) {
  			const typeNames = Object.keys( inheritFrom ).filter( name => name.startsWith( 'is' ) );
  
  			for ( const name of typeNames ) {
  				if ( !( name in item ) ) {
  					item[ name ] = inheritFrom[ name ];
  				}
  			}
  		}
  	}
  
  	delete item.inheritTypesFrom;
  }
  
  // Remove items which weren't registered (because it may break some checks or we'd need to complicate them).
  // Make sure allowIn doesn't contain repeated values.
  function cleanUpAllowIn( compiledDefinitions, itemName ) {
  	const itemRule = compiledDefinitions[ itemName ];
  	const existingItems = itemRule.allowIn.filter( itemToCheck => compiledDefinitions[ itemToCheck ] );
  
  	itemRule.allowIn = Array.from( new Set( existingItems ) );
  }
  
  // Setup allowChildren items based on allowIn.
  function setupAllowChildren( compiledDefinitions, itemName ) {
  	const itemRule = compiledDefinitions[ itemName ];
  
  	for ( const allowedParentItemName of itemRule.allowIn ) {
  		const allowedParentItem = compiledDefinitions[ allowedParentItemName ];
  
  		allowedParentItem.allowChildren.push( itemName );
  	}
  }
  
  function cleanUpAllowAttributes( compiledDefinitions, itemName ) {
  	const itemRule = compiledDefinitions[ itemName ];
  
  	itemRule.allowAttributes = Array.from( new Set( itemRule.allowAttributes ) );
  }
  
  function copyTypes( sourceItemRules, itemRule ) {
  	for ( const sourceItemRule of sourceItemRules ) {
  		const typeNames = Object.keys( sourceItemRule ).filter( name => name.startsWith( 'is' ) );
  
  		for ( const name of typeNames ) {
  			itemRule[ name ] = sourceItemRule[ name ];
  		}
  	}
  }
  
  function copyProperty( sourceItemRules, itemRule, propertyName ) {
  	for ( const sourceItemRule of sourceItemRules ) {
  		if ( typeof sourceItemRule[ propertyName ] == 'string' ) {
  			itemRule[ propertyName ].push( sourceItemRule[ propertyName ] );
  		} else if ( Array.isArray( sourceItemRule[ propertyName ] ) ) {
  			itemRule[ propertyName ].push( ...sourceItemRule[ propertyName ] );
  		}
  	}
  }
  
  function makeInheritAllWork( sourceItemRules, itemRule ) {
  	for ( const sourceItemRule of sourceItemRules ) {
  		const inheritFrom = sourceItemRule.inheritAllFrom;
  
  		if ( inheritFrom ) {
  			itemRule.allowContentOf.push( inheritFrom );
  			itemRule.allowWhere.push( inheritFrom );
  			itemRule.allowAttributesOf.push( inheritFrom );
  			itemRule.inheritTypesFrom.push( inheritFrom );
  		}
  	}
  }
  
  function getAllowedChildren( compiledDefinitions, itemName ) {
  	const itemRule = compiledDefinitions[ itemName ];
  
  	return getValues( compiledDefinitions ).filter( def => def.allowIn.includes( itemRule.name ) );
  }
  
  function getValues( obj ) {
  	return Object.keys( obj ).map( key => obj[ key ] );
  }
  
  function mapContextItem( ctxItem ) {
  	if ( typeof ctxItem == 'string' || ctxItem.is( 'documentFragment' ) ) {
  		return {
  			name: typeof ctxItem == 'string' ? ctxItem : '$documentFragment',
  
  			* getAttributeKeys() {},
  
  			getAttribute() {}
  		};
  	} else {
  		return {
  			// '$text' means text nodes and text proxies.
  			name: ctxItem.is( 'element' ) ? ctxItem.name : '$text',
  
  			* getAttributeKeys() {
  				yield* ctxItem.getAttributeKeys();
  			},
  
  			getAttribute( key ) {
  				return ctxItem.getAttribute( key );
  			}
  		};
  	}
  }
  
  // Generator function returning values from provided walkers, switching between them at each iteration. If only one walker
  // is provided it will return data only from that walker.
  //
  // @param {module:engine/module/treewalker~TreeWalker} [backward] Walker iterating in backward direction.
  // @param {module:engine/module/treewalker~TreeWalker} [forward] Walker iterating in forward direction.
  // @returns {Iterable.<Object>} Object returned at each iteration contains `value` and `walker` (informing which walker returned
  // given value) fields.
  function* combineWalkers( backward, forward ) {
  	let done = false;
  
  	while ( !done ) {
  		done = true;
  
  		if ( backward ) {
  			const step = backward.next();
  
  			if ( !step.done ) {
  				done = false;
  				yield {
  					walker: backward,
  					value: step.value
  				};
  			}
  		}
  
  		if ( forward ) {
  			const step = forward.next();
  
  			if ( !step.done ) {
  				done = false;
  				yield {
  					walker: forward,
  					value: step.value
  				};
  			}
  		}
  	}
  }
  
  // Takes an array of non-intersecting ranges. For each of them gets minimal flat ranges covering that range and returns
  // all those minimal flat ranges.
  //
  // @param {Array.<module:engine/model/range~Range>} ranges Ranges to process.
  // @returns {Iterable.<module:engine/model/range~Range>} Minimal flat ranges of given `ranges`.
  function* convertToMinimalFlatRanges( ranges ) {
  	for ( const range of ranges ) {
  		yield* range.getMinimalFlatRanges();
  	}
  }
  
  function removeDisallowedAttributeFromNode( schema, node, writer ) {
  	for ( const attribute of node.getAttributeKeys() ) {
  		if ( !schema.checkAttribute( node, attribute ) ) {
  			writer.removeAttribute( attribute, node );
  		}
  	}
  }
  