Generality of Unifiers

Some pairs of literals may have more than one possible unifier.

P(x), P(y) is unified by each of { x/y }, { y/x },

{ z/x, z/y }, { a/x, a/y}, { f(a)/x, f(a)/y } etc.

Suppose S1 and S2 are unifiers for L1 and 12.

Then S1(P1) = S1(P2) or S1(P1) = ~S1(P2)

S2(P1) = S2(P2) or S2(P1) = ~S2(P2)

S2 is less general than S1 if no S3 exists such that

S3(S2(P1)) = S1(P1).

{ a/x, a/y } is less general than { y/x }.

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