docs: function Vector2D(x, y) {
          if (x instanceof Vector2D) {
              this.x = x.x;
              this.y = x.y;
          } else if (arguments.length === 2) {
              this.x = x;
              this.y = y;
          } else if (arguments.length > 0)
              throw "Unexpected number of arguments for Vector2D()";
      }