apply_func_to_leaf_subtrees
===========================

.. py:function:: t3toolbox.backend.stacking.apply_func_to_leaf_subtrees(tree, func, leaf_structure)

   .. code-block:: python

      def apply_func_to_leaf_subtrees(
              tree,                  # nested tuples; each leaf is itself a subtree shaped like leaf_structure
              func: typ.Callable,    # applied to each leaf-subtree
              leaf_structure,        # nested-tuple template defining what counts as one leaf
      ):


   Apply a function to all "leafs" in a tree.
   A "leaf" is, itself, a subtree with the structure given in leaft_structure.

   .. rubric:: Examples

   >>> import numpy as np
   >>> import t3toolbox.backend.stacking as stacking
   >>> func = lambda x: (x[0] - x[1][0], x[0] + x[1][1]) # (a,(b,c)) -> (a-b, a+c)
   >>> LS = (None, (None, None))
   >>> T1 = (1, (2, 3))
   >>> T2 = (4, (5, 6))
   >>> T3 = (7, (8, 9))
   >>> T = ((T1, T2), ((T3,),))
   >>> print(stacking.apply_func_to_leaf_subtrees(T, func, LS))
   (((-1, 4), (-1, 10)), (((-1, 16),),))