Safety and binding

A variable is bound by a positive predicate atom, or by an equality X = e once every variable in e is bound.

Other comparisons (<, >, !=, …) and negated atoms only test variables; they never bind them.

A rule is safe when every variable in the head, in a comparison, and under not is bound.

• ✓ divisor(D, N) :- num(D), num(M), D > 1, M > 1, N = D * M: num binds D and M, then the equality N = D * M binds N.
• ✗ prime(N) :- not composite(N): N appears only under not, so it is never bound.