code: function (v, alpha) {
              this._x += ( v.x - this._x ) * alpha;
              this._y += ( v.y - this._y ) * alpha;
              return this;
          }
docs: Linearly interpolate between this vector and the given one.
  @name lerp
  @memberOf me.ObservableVector2d
  @function
  @param {me.Vector2d|me.ObservableVector2d} v
  @param {Number} alpha distance along the line (alpha = 0 will be this vector, and alpha = 1 will be the given one).
  @return {me.ObservableVector2d} Reference to this object for method chaining