code: function (angle) {
              var x = this._x;
              var y = this._y;
              return this._set(x * Math.cos(angle) - y * Math.sin(angle), x * Math.sin(angle) + y * Math.cos(angle));
          }
docs: Rotate this vector (counter-clockwise) by the specified angle (in radians).
  @name rotate
  @memberOf me.ObservableVector2d
  @function
  @param {number} angle The angle to rotate (in radians)
  @return {me.ObservableVector2d} Reference to this object for method chaining