What does the following expression mean? all_of(x) != any_of(y) 1) It could mean that each of the values in x must not be equal to any one of the values in y. OTOH, should that expression be symmetrical? any_of(y) != all_of(x) 2) When expressed that way, it suggests that as long as any of the values in y is not equal to all of the values in x, the result is true. That is, given the following sets of values s1 = { 1, 2, 3 } s2 = { 3, 4, 5 } what should be the value of this expression? all_of(s1) != any_of(s2) Intuitively, that seems to suggest 1), which means it evaluates to false. When reversed any_of(s2) != all_of(s1) the expression seems to suggest 2), which means it evaluates to true. I don't like the idea of these expressions being asymmetrical. Perhaps we need each_of to mean each value should be compared against the other side and, provided all of those comparisons succeed, the expression evaluates to true. Then, all_of can mean that all of the values must satisfy the *same* comparison. Put another way, all_of(x) op any_of(y) must be implemented by iterating y and comparing against all_of(x). With those two types, and given s1 and s2 from above, we'd have the following expressions evaluate to true: all_of(s1) != any_of(s2) !(each_of(s1) != any_of(s2)) -- Rob Stewart stewart@sig.com Software Engineer http://www.sig.com Susquehanna International Group, LLP using std::disclaimer;