code: function(a,b) {
  			var v = _getXY(a,b);
  			this.x -= v.x;
  			this.y -= v.y;
  			return this;
  		}
docs: Subtracts vector {a,b,c} and overrides coordinates with result.
  
  @param a
  X coordinate
  @param b
  Y coordinate
  @return itself