Unifiers

Given a pair of literals L1 and L2, if S is a substitution such that

S(L1) = S(L2) or S(L1) = ~ S(L2),

then S is a unifier for L1 and L2.

Example:

L1 = ~P(x, f(a))

L2 = P(b, y)

S = { b/x, f(a)/y }

S(L1) = ~P(b, f(a)) = ~S(L2)

Therefore S is a unifier for L1 and L2.

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