anchor.diff:     * Returns the value bound to [key].
      * 
      * If [key] is not bound, [orElse] is called to obtain the
  -   * return value. Default [orElse] throws [RangeError].
  +   * return value. Default [orElse] throws exception.
      */
     V lookup(K key, {orElse()});
   
  
positive.code:   /**
     * Returns the value bound to [key].
     * 
     * Throws [RangeError] if [key] is not bound.
     */
    V operator [](K key);
  
  
negative.code:   /**
     * Unbinds [key].
     *
     * If [key] is not bound and [safe] is not `true`, exception is thrown.
     * If [key] is not bound and [safe] is specified as `true`, nothing happens.
     *
     *     var map = PersistentMap.fromMap({'a': 1, 'b': 2}).asTransient();
     *     map.doDelete('b', 2); // map is now {'a': 1}
  