ufv_variations_sum_stack
========================

.. py:function:: t3toolbox.backend.ufv_operations.ufv_variations_sum_stack(data, axis = None)

   .. code-block:: python

      def ufv_variations_sum_stack(
              data,             # UT3Variations .data: (tkv, ttv, shape, (4 masks))
              axis: typ.Optional[int] = None,  # stack axis to sum (None = whole stack); 0-based within the stack
      ):  # -> summed .data (the summed stack axes removed)


   Sum a UT3Variations over stack axes (a batch of tangents -> their sum; corewise == tangent sum by
   linearity). The supercores sum via ``xnp``; the masks **OR** over the same axes (host ``np``) -- the
   union of real slots, a no-op for a same-mask stack (``docs/uniform_masks_vs_ranks.md``). The stack
   lives at axes ``1 ..`` (after the leading mode index ``d``), shared by supercores and masks.