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