text: 
  /**
  @class 2D vector
  */
  function Vector2(x, y)
  {
      this.x = x;
      this.y = y;
  }
  /**
  Add two vectors
  */
  Vector2.add = function (v1, v2)
  {
      return new Vector2(v1.x + v2.x, v1.y + v2.y);
  }
  Vector2.prototype.add = function (v)
  {
      return new Vector2(this.x + v.x, this.y + v.y);
  }
  