Sum and Product have a indexing variable which is a Atom but not a free
symbol. So logic, that defines the undefined symbols in a SympyAssignment should not be
atoms(sp.Symbol) but free_symbols. sp.Indexed from the ResolvedFieldAcesses forms an edge case.
So we could also use atoms(sp.Symbol).intersection(...free_symbols).
I hope I extracted from my fork all the necessary code to implement this feature.