This may require supporting "basis" as a list of basis vectors
(as opposed to superalgebra elements) in the subalgebra constructor.
+
+8. Implement random_instance() for general algebras as random_eja().
+ Copy/paste the "general" construction into the other classes that
+ can use it. The general construction can be something like "call
+ random_instance() on something that inherits me and return the
+ result."