+ We can't give reliable answers over inexact rings::
+
+ sage: K = Cone([(1,2,3), (4,5,6)])
+ sage: L = identity_matrix(RR,3)
+ sage: is_cross_positive_on(L,K)
+ Traceback (most recent call last):
+ ...
+ ValueError: The base ring of L is neither SR nor exact.
+