non_stratification.m:017: In `foo'(in):
non_stratification.m:017:   warning: call introduces a non-stratified loop.
non_stratification.m:017:   A non-stratified loop is a loop in the call graph
non_stratification.m:017:   of the given predicate/function that allows it to
non_stratification.m:017:   call itself in a negated context. This can cause
non_stratification.m:017:   problems for bottom-up evaluation of the
non_stratification.m:017:   predicate/function.