Unification
	l To unify Knows(John,x) and Knows(y,z),    θ = {y/John, x/z } or θ = {y/John, x/John, z/John} l The first unifier is more general than the second. l There is a single most general unifier (MGU) that is unique up to renaming of variables. MGU = { y/John, x/z }