-6. Override inner_product(), _max_test_case_size(), et cetera in
- DirectSumEJA.
-
-7. Switch to QQ in *all* algebras for _charpoly_coefficients().
+ sage: J1 = ComplexHermitianEJA(2)
+ sage: J2 = TrivialEJA()
+ sage: J = cartesian_product([J1,J2])
+ sage: x = J.from_vector(vector(QQ, [-1, -1/2, -1/2, -1/2]))
+ sage: x.is_invertible()
+ True
+ sage: x.det()
+ 0