anchor.diff:    /**
      * Returns the value bound to [key].
      * 
  -   * Throws [RangeError] if [key] is not bound.
  +   * Throws exception if [key] is not bound.
      */
     V operator [](K key);
   
  
positive.code:   /**
     * Returns a new map identical to `this` except that the value it possibly
     * binds to [key] has been adjusted by [update].
     *
     *     {'a': 1, 'b': 2}.adjust('b', (x) => x + 1) == {'a': 1, 'b': 3}
     *     {'a': 1}.adjust('b', (x) => x + 1) == {'a': 1}
  
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}
  