+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).