-A. Make the subalgebra class work with any subalgebra (there's nothing
- special except a_regular_element() in there).
+1. Finish CartesianProductEJA: add to_matrix(), random_instance(),...
+ methods. I guess we should create a separate class hierarchy for
+ Cartesian products of RationalBasisEJA? That way we get fast
+ charpoly and random_instance() defined...