anchor.diff:     */
     Optional<dynamic> map(dynamic mapper(T val)) {
       if (isPresent) return new Optional<dynamic>.ofNullable(mapper(_value));
  -    else return new Optional<dynamic>.empty();
  +    else return EMPTY;
     }
   
     /// Returns this Optional's value, if present.  Otherwise, returns other.
  
positive.code:  * Offers several methods which depend on the presence or absence of a contained value.
   */
  class Optional<T> {
  
    final T _value;
  
  
negative.code:   /// Returns an Optional with this Optional's value, if there is a value present and it matches the predicate.  Otherwise, returns an empty Optional.
    Optional<T> filter(bool predicate(T val)) {
      if (isPresent && predicate(_value)) return new Optional<T>.of(_value);
      else return EMPTY;
    }
  
    /// Returns an Optional provided by applying the mapper to this Optional's value, if present.  Otherwise, returns an empty Optional.
    Optional<dynamic> flatMap(Optional<dynamic> mapper(T val)) {
      if (isPresent) return mapper(_value);
      else return EMPTY;
    }
  
    /**
  