[dune-functions] Access sub-preBasis of PowerBasis

[Lasse Hinrichsen]

Dear dune-functions experts,

I was wondering how to access the underlying PreBasis of a power basis
(similarly, for a composite).

Say, I have some code like this:

auto basis = makeBasis(
  grid->leafGridView(),
  power<N>(
   my_fancy_prebasis(),
   blockedInterleaved()
  )
);

and `my_fancy_prebasis` contains something I still need at runtime (say,
the order of ansatz functions which I may want to change during runtime).

Is there a way to access this (sub-)PreBasis? Clearly, the PowerPreBasis
holds it as a member, but it does not provide access to it. Of course,
one could easily provide access there, but I wonder, if

a) there is another way I didnt see,

or alternatively

b) the approach is wrong anyway and I (the user) am not supposed to
touch the PreBasis ever again (if so, what would be a more canonical way
to handle such a situation?).

Thank you in advance for all suggestions!

Best,
Lasse Hinrichsen