docs: def inv(self):
          """Invert the transform.
  
          After calling this method, calling the instance will do the inverse
          transform. Calling this twice return the instance to the original
          transform.
          """
  
          self.x, self.y = self.y, self.x
          self._x_, self._y_ = self._y_, self._x_
          self.xfac, self.yfac = 1 / self.yfac, 1 / self.xfac
          self._xfac_, self._yfac_ = 1 / self._yfac_, 1 / self._xfac_
          self._u = 1 / self._u.conj()