add debug statement for cdf failure

The arguments of Min are stored in a frozenset and
during subs, the hashable content (the frozenset) is
checked. It is my understanding that frozensets should
compare equal regarless of the order in which the arguments
are given, so it is unclear how the debug statement will
ever pass...but a test run shows that the cdf(X)(z) at line
2217 in crv_types sometimes fails, allowing Min(z, a) to pass
through the substitution.