anchor.diff:    /**
      * Returns a new map identical to `this` except that it doesn't bind [key]
      * anymore.
  +   * 
  +   * If [key] is not bound and [safe] is not `true`, exception is thrown.
  +   * If [key] is not bound and [safe] is specified as `true`,
  +   * the same map is returned. 
      *
      *     {'a': 1, 'b': 2}.delete('b') == {'a': 1}
      *     {'a': 1}.delete('b') == {'a': 1}
  
positive.code:   /**
     * Returns the value bound to [key].
     * 
     * Throws [RangeError] if [key] is not bound.
     */
    V operator [](K key);
  
  
negative.code:    * Returns the value bound to [key].
     * 
     * If [key] is not bound, [orElse] is called to obtain the
     * return value. Default [orElse] throws exception.
     */
    V lookup(K key, {orElse()});
  
  