Substitutions
A substitution is a set of term/variable pairs.
e.g., { a/x, f(a)/y, w/z }
Each element of the set is an elementary substitution.
A particular variable occurs at most once on the the right-hand side of any elementary subst. in a substitution.
{ a/x, b/x } is not an acceptable substitution.
The empty set is an acceptable substitution.
If E is a term or a formula of the predicate calculus, and S is a substitution, then S(E) is the result of applying S to E, i.e., replacing each variable of S that occurs in E by the corresponding term in S.