
      return new Point(this.x, this.y);
    };
    Point.prototype.length = function(length) {
      if (typeof length == 'undefined')
        return Math.sqrt(this.x * this.x + this.y * this.y);
      this.normalize();
      this.x *= length;
      this.y *= length;
      return this;
    };

    Point.prototype.normalize = function() {
      var length = this.length();
      this.x /= length;
      this.y /= length;
      return this;
    };
    return Point;
  })();

  var Particle 