Unification of Literals
A substitution is a set of term/variable pairs.
A unifier for a pair of literals is a substitution that when applied to both literals, makes them identical.
P(x, a), P(f(a), y) have the unifier
P(x), P(y) have the unifier { a/x, a/y },
but they also have the unifier { x/y }.
The latter is more general because after
unifying with { x/y} we get P(x) whereas with the other it is P(a), yet we can obtain the latter from the former with an additional substitution { a/x }.