coefficients as polynomials

This question [1] involves a polynomial in two sets of variables where the question asks how to extract the coefficients of one set as polynomials in the other set. Solutions are shown there using mpoly and sympy (Python) and there are links to solutions using the R package caracas [2] and julia [3]. These may suggest features that could be added to mpoly.

[1] https://stackoverflow.com/questions/72996803/substitute-the-variables-of-a-polynomial-with-caracas-sympy/72998576

[2] https://laustep.github.io/stlahblog/posts/caracas01.html
https://laustep.github.io/stlahblog/posts/caracas02.html

[3] https://laustep.github.io/stlahblog/posts/juliaPolynomialExpansion.html