code: public void flipY(double y0, double y1) {
  		yx = -yx;
  		yy = -yy;
  		yd = y0 + y1 - yd;
  	}
docs: Flips the transformation around the Y axis.
  
  @param y0
  The Y coordinate to flip.
  @param y1
  The Y coordinate to flip to.