Simple Prolog

Implementation of Prolog ProL language and WIM virtual machine. Based on W. Reinhard and H. Seidl "Compiler Design, Virtual Machines". Please let me know what you think.

Ex 1. Towers of Hanoi

move(N,X,Y,Z) <= N = 1, Z = _, write(move), write(X), write(to), write(Y), nl
move(N,X,Y,Z) <= N > 1,
                M is N - 1,
                move(M,X,Z,Y),
                move(1,X,Y,_),
                move(M,Z,Y,X)
    <= move(3,source,target,auxiliary)

move source to target
move source to auxiliary
move target to auxiliary
move source to target
move auxiliary to source
move auxiliary to target
move source to target

Ex 2. List Concatenation

list_concat(L1, L2, L3) <= L1 = [], L2 = L3
list_concat(L1, L2, L3) <= L1 = [H1|T1], L3 = [H1|T3], list_concat(T1,L2,T3)
    <= list_concat([a, b, c], [d, e, f], Result)

[a, b, c, d, e, f]
-----------------
no

Ex 3. List Permutations

list_delete(X, P1, L1) <= P1 = [X|L1]
list_delete(X, P1, P2) <= P1 = [Y|L2], P2 = [Y|L1], list_delete(X, L2, L1)

list_perm(L, P2) <= L = [], P2 = []
list_perm(L, P2) <= P2 = [X|P], list_delete(X, L, L1), list_perm(L1, P)

    <= list_perm([a, b, c], X)

[a, b, c]
-----------------
[a, c, b]
-----------------
[b, a, c]
-----------------
[b, c, a]
-----------------
[c, a, b]
-----------------
[c, b, a]
-----------------
no

Ex 4. Cut Operator

Without cut.

f(X) <= X = a
f(X) <= X = b
f(X) <= X = c
    <= f(X), f(Y)

a a
-----------------
a b
-----------------
a c
-----------------
b a
-----------------
b b
-----------------
b c
-----------------
c a
-----------------
c b
-----------------
c c
-----------------
no

With cut.

f(X) <= X = a
f(X) <= X = b
f(X) <= X = c
 <= f(X), !, f(Y)

a a
-----------------
a b
-----------------
a c
-----------------
no

Ex 5. Facts Database

parent(X, Y) <= X = slawek, Y = kuba
parent(X, Y) <= X = slawek, Y = maks
parent(X, Y) <= X = boguslaw, Y = slawek
parent(X, Y) <= X = boguslaw, Y = beata
parent(X, Y) <= X = elzbieta, Y = slawek
parent(X, Y) <= X = elzbieta, Y = beata

grandparent(X,Y) <= parent(X, Z),
                    parent(Z, Y)

<= grandparent(X, Y)

boguslaw
kuba
-----------------
boguslaw
maks
-----------------
elzbieta
kuba
-----------------
elzbieta
maks
-----------------
no