anchor.diff:      return _value;
     }
   
  -  /**
  -   * Executes a function if the Optional value is present.
  -   */
  +  /// Executes a function if the Optional value is present.
     void ifPresent(void ifPresent(T value)) {
       if (isPresent) {
         ifPresent(_value);
       }
     }
   
  -  /**
  -   * Execution a function if the Optional value is absent.
  -   */
  +  /// Execution a function if the Optional value is absent.
     void ifAbsent(void ifAbsent()) {
       if (!isPresent) {
         ifAbsent();
       }
     }
   
  -  /**
  -   * Gets the Optional value with a default.
  -   *
  -   * The default is returned if the Optional is [absent()].
  -   *
  -   * Throws [ArgumentError] if [defaultValue] is null.
  -   */
  +  /// Gets the Optional value with a default.
  +  ///
  +  /// The default is returned if the Optional is [absent()].
  +  ///
  +  /// Throws [ArgumentError] if [defaultValue] is null.
     T or(T defaultValue) {
       if (defaultValue == null) {
         throw new ArgumentError('defaultValue must not be null.');
  
positive.code: 
  library quiver.optional;
  
  /**
   * A value that might be absent.
   *
   * Use Optional as an alternative to allowing fields, parameters or return
   * values to be null. It signals that a value is not required and provides
   * convenience methods for dealing with the absent case.
   */
  class Optional<T> {
    final T _value;
  
    /**
     * Constructs an empty Optional.
     */
    const Optional.absent() : _value = null;
  
    /**
     * Constructs an Optional of the given [value].
     *
     * Throws [ArgumentError] if [value] is null.
     */
    Optional.of(T value) : this._value = value {
      if (this._value == null) throw new ArgumentError('Must not be null.');
    }
  
    /**
     * Constructs an Optional of the given [value].
     *
     * If [value] is null, returns [absent()].
     */
    const Optional.fromNullable(T value) : this._value = value;
  
    /**
     * Whether the Optional contains a value.
     */
    bool get isPresent => _value != null;
  
    /**
     * Gets the Optional value.
     *
     * Throws [StateError] if [value] is null.
     */
    T get value {
      if (this._value == null) {
        throw new StateError('value called on absent Optional.');
  
negative.code:     return _value == null ? defaultValue : _value;
    }
  
    /// Gets the Optional value, or [null] if there is none.
    T get orNull => _value;
  
    /// Transforms the Optional value.
    ///
    /// If the Optional is [absent()], returns [absent()] without applying the transformer.
    ///
    /// The transformer must not return [null]. If it does, an [ArgumentError] is thrown.
    Optional transform(dynamic transformer(T value)) {
      return _value == null
          ? new Optional.absent()
          : new Optional.of(transformer(_value));
    }
  
    /// Delegates to the underlying [value] hashCode.
    int get hashCode => _value.hashCode;
  
    /// Delegates to the underlying [value] operator==.
    bool operator ==(o) => o is Optional && o._value == _value;
  
    String toString() {
  