-7. If we factor out a "matrix algebra" class, then it would make sense
- to replace the custom embedding/unembedding functions with static
- _real_embedding() and _real_unembedding() methods.
+5. Compute the scalar in the general natural_inner_product() for
+ matrices, so no overrides are necessary. Actually, this is
+ probably better implemented as a dimension_over_reals() method
+ that returns 1, 2, or 4.
+
+6. The main EJA element constructor is happy to convert between
+ e.g. HadamardEJA(3) and JordanSpinEJA(3).