-A. Make the subalgebra class work with any subalgebra (there's nothing
- special except a_regular_element() in there).
+1. Finish DirectSumEJA: add to_matrix(), random_instance(),
+ one()... methods. Make it subclass RationalBasisEuclideanJordanAlgebra.
+ This is not a general direct sum / cartesian product implementation,
+ it's used only with the other rationalbasis algebras (to make non-
+ simple EJAs out of the simple ones).