Unification Algorithm

Given two predicates that initially have no (universally quantified) variables in common, a unification algorithm should:

• Find a substitution of terms for variables that will make the two predicates identical, or

• Report that no such substitution exists:
  the predicates do not unify.

user=> (unify '(p  x   (a))
              '(p (b)   y )   )

( (y (a))   (x (b))   (t t))

user=> (unify '(q (a))
              '(q (b)))
nil