docs: def clip(self, x1, y1, x2, y2):
          """Activate a rectangular clip region, (X1, Y1) - (X2, Y2).
          You must call endclip() after you completed drawing.
  
  canvas.clip(x,y,x2,y2)
  draw something ...
  canvas.endclip()
  """
  
          self.__clip_stack.append(self.__clip_box)
          self.__clip_box = _intersect_box(self.__clip_box, (x1, y1, x2, y2))
          self.gsave()
          self.newpath()
          self.moveto(xscale(x1), yscale(y1))
          self.lineto(xscale(x1), yscale(y2))
          self.lineto(xscale(x2), yscale(y2))
          self.lineto(xscale(x2), yscale(y1))
          self.closepath()
          self.clip_sub()