Safe calculus queries if (from textbook):
For any I, answers for Q contain only values in Dom(Q, I), where Dom(Q, I) is the set of all constants in relational instance I or query Q
Goal is to insure finite result
For each "EXISTS R (p(R))" in Q, if for tuple r, p(r) true, then elements of r in Dom(Q,I)
insuring the checking of "EXISTS" is finite
For each "FOR ALL R (p(R))" in Q, if tuple r contains a constant not in Dom(Q,I), then p(r) true
insuring the checking of "FOR ALL" is finite