code: function (v) {
              return Math.sqrt((this._x - v.x) * (this._x - v.x) + (this._y - v.y) * (this._y - v.y));
          }
docs: return the distance between this vector and the passed one
  @name distance
  @memberOf me.ObservableVector2d
  @function
  @param {me.ObservableVector2d} v
  @return {Number}