Algorithm: Unify(Ψ1, Ψ2) Step. 1: If Ψ1 or Ψ2 is a variable or constant, then: a) If Ψ1 or Ψ2 are identical, then return NIL. b) Else if Ψ1is a variable, a. then if Ψ1 occurs in Ψ2, then return FAILURE b. Else return { (Ψ2/ Ψ1)}. c) Else if Ψ2 is a variable, a. If Ψ2 occurs in Ψ1 then return FAILURE, b. Else return {( Ψ1/ Ψ2)}. d) Else return FAILURE. Step.2: If the initial Predicate symbol in Ψ1 and Ψ2 are not same, then return FAILURE. Step. 3: IF Ψ1 and Ψ2 have a different number of arguments, then return FAILURE. Step. 4: Set Substitution set(SUBST) to NIL. Step. 5: For i=1 to the number of elements in Ψ1. a) Call Unify function with the ith element of Ψ1 and ith element of Ψ2, and put the result into S. b) If S = failure then returns Failure c) If S ≠ NIL then do, a. Apply S to the remainder of both L1 and L2. b. SUBST= APPEND(S, SUBST). Step.6: Return SUBST.