It seems to me that the author isn't comfortable with static typing. The obvious solution (which might be impossible in MyPy?) is to have a two-parameter type c

It seems to me that the author isn't comfortable with static typing. The obvious solution (which might be impossible in MyPy?) is to have a two-parameter type class (protocol in MyPy?) of the lhs and rhs, and have the output be of an associated type. That should cover almost all cases.