Ah, yes! There are constructions of parents where the second argument is
not the base ring, and there are also constructions with higher arity.
So, I guess that, to be on the safe side, a generic change_ring method
would have to check `self.construction()[1] == self.base_ring()`.
Martin
On Sat
Dear Martin,
I don't know much about this area but I don't think construction() is
always implemented like that. For example FreeAlgebra(QQ,
2).tensor(FreeAlgebra(QQ, 2)).construction() returns a functor and two
arguments.
As a default implementation in the category this might still well work f
