-1. Finish CartesisnProductEJA: add to_matrix(), random_instance(),
- one()... methods. This will require rethinking what a "matrix
- representation" and "matrix space" means for a cartesian product
- algebra. Do we want our matrix basis to consist of ordered pairs
- (or triples, or...)? Should the matrix_space() of the algebra
- be the cartesian product of the factors' matrix spaces? Can
- the FDEJA initializer be made to work on tuples, or will it
- need to be overridden?