ufv_variation_masks
===================

.. py:function:: t3toolbox.backend.ufv_masking.ufv_variation_masks(frame_masks)

   .. code-block:: python

      def ufv_variation_masks(
              frame_masks:  typ.Tuple[NDArray, NDArray, NDArray, NDArray],  # (up, down, frame_left, frame_right); HOST bool
      ) -> typ.Tuple[NDArray, NDArray, NDArray, NDArray]:                   # (up, down, left[:-1], right[1:])


   The variation masks of a frame: its rank masks with the **gauge shift** applied.

   A variation occupies one TT slot, not a boundary edge, so it takes the frame's left masks without
   the last and the right masks without the first. The up/down masks pass through. This one line is
   the whole convention, and it was previously open-coded at four sites; call it rather than slicing
   by hand. (The frontend twin is ``UT3Variations._variation_masks_of``.)