code: function(theta) {
  			var co = Math.cos(theta);
  			var si = Math.sin(theta);
  			var xx = co * this.x - si * this.y;
  			this.y = si * this.x + co * this.y;
  			this.x = xx;
  			return this;
  		}
docs: Rotates the vector by the given angle around the Z axis.
  
  @param theta
  @return itself