ut3_sum_stack
=============

.. py:function:: t3toolbox.backend.ut3_linalg.ut3_sum_stack(x)

   .. code-block:: python

      def ut3_sum_stack(x: UT3Data) -> UT3Data:  # sum over ALL stack axes -> unstacked UT3 (NOT squashed)


   Sum the represented dense tensors over the whole stack (the genuine tensor sum, not a corewise
   sum): fold the stack ``S`` into the Tucker rank (merge) and into the TT ranks (block-diagonal over
   all three TT axes via three identities), and likewise reshape the masks. The frontend then squashes
   the tails, which performs the summation. ``S``-fold generalization of :py:func:`ut3_add`.