check_perturbation_index
========================

.. py:function:: t3toolbox.backend.sampling_derivatives.check_perturbation_index(index, pp, shape = None)

   .. code-block:: python

      def check_perturbation_index(
              index: NDArray,                          # grid points, int, shape=(d,)+W
              pp:    typ.Sequence[NDArray],            # perturbation P, len=d, elm_shape=W+(Ni,)
              shape: typ.Optional[typ.Sequence[int]]   # ambient mode dims (Ni), len=d -- if given, also check P's mode dim
                   = None,
      ) -> None:


   Structural check (hard error): the perturbation ``P`` shares the sample stack ``W`` of the grid
   points ``index`` (shape ``(d,)+W``), and -- when ``shape`` is given (the ambient mode dims ``Ni``,
   which the integer ``index`` does not carry) -- ``P``'s mode dim matches it. Used by the
   derivative-entries frontends; jit-safe.