code: ///////////////////////////////////////////////////////////////////////////
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  //                 / /   / __ \/ __ `/ / ___/  |/ / / __                 //
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  //                                                                       //
  //               The Next Generation Logic Library                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  //                                                                       //
  //  Copyright 2015-20xx Christoph Zengler                                //
  //                                                                       //
  //  Licensed under the Apache License, Version 2.0 (the "License");      //
  //  you may not use this file except in compliance with the License.     //
  //  You may obtain a copy of the License at                              //
  //                                                                       //
  //  http://www.apache.org/licenses/LICENSE-2.0                           //
  //                                                                       //
  //  Unless required by applicable law or agreed to in writing, software  //
  //  distributed under the License is distributed on an "AS IS" BASIS,    //
  //  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or      //
  //  implied.  See the License for the specific language governing        //
  //  permissions and limitations under the License.                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  
  package org.logicng.io.parsers;
  
  import org.antlr.v4.runtime.CommonTokenStream;
  import org.logicng.formulas.FormulaFactory;
  
  /**
   * A parser for propositional formulas.
   * <p>
   * The syntax for propositional formulas in LogicNG is:
   * <ul>
   * <li>{@code $true} for the constant TRUE</li>
   * <li>{@code $false} for the constant FALSE</li>
   * <li>{@code ~} for the negation</li>
   * <li>{@code |} for the disjunction (OR)</li>
   * <li>{@code &} for the conjunction (AND)</li>
   * <li>{@code =>} for the implication</li>
   * <li>{@code <=>} for the equivalence</li>
   * </ul>
   * Brackets are {@code (} and {@code )}.  For variable names, there are the following rules:
   * <ul>
   * <li>must begin with a alphabetic character, {@code _}, {@code @}, or {@code #}</li>
   * <li>can only contain alphanumerical character, {@code _}, or {@code #}</li>
   * <li>{@code @} is only allowed at the beginning of the variable name and is reserved for special internal variables</li>
   * </ul>
   * @version 2.4.1
   * @since 1.0
   */
  public final class PropositionalParser extends FormulaParser {
  
      /**
       * Constructs a new parser.
       * @param f the formula factory
       */
      public PropositionalParser(final FormulaFactory f) {
          super(f);
          final PropositionalLexer lexer = new PropositionalLexer(null);
          final CommonTokenStream tokens = new CommonTokenStream(lexer);
          final ParserWithFormula parser = new LogicNGPropositionalParser(tokens);
          setLexerAndParser(lexer, parser);
      }
  }
  