Weak empty multisets contaminate other empty multisets
Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1728
Reported by Fredrik Noring, Roxen Internet Software noring@roxen.com
> multiset a = (< >);
Result: (< >)
> multiset b = set_weak_flag( (< >), 1);
Result: (< >)
> get_weak_flag(a);
Result: 1