diff --git a/implementations/GNUProlog/parser.prolog b/implementations/GNUProlog/parser.prolog
index 256043c..9efcebb 100644
--- a/implementations/GNUProlog/parser.prolog
+++ b/implementations/GNUProlog/parser.prolog
@@ -1,4 +1,5 @@
 
+:- dynamic(def/2).
 
 % For number_codes/2 we want to just fail if the codes do not represent an integer.
 % gprolog.html#number-atom%2F2
@@ -6,6 +7,12 @@
 :- set_prolog_flag(syntax_error, fail).
 
 
+joy(InputString, StackIn, StackOut) :-
+    text_to_expression(InputString, Expression),
+    !,
+    thun(Expression, StackIn, StackOut).
+
+
 joy_lex([tok(Token)|Ls]) --> chars(Token), !, joy_lex(Ls).
 joy_lex([  lbracket|Ls]) --> "[",          !, joy_lex(Ls).
 joy_lex([  rbracket|Ls]) --> "]",          !, joy_lex(Ls).
@@ -24,7 +31,7 @@ joy_parse([]) --> [].
 joy_term(list(J)) --> [lbracket], !, joy_parse(J), [rbracket].
 joy_term(Token) --> [tok(Codes)], {joy_token(Token, Codes)}.
 
-joy_token(int(I), Codes) :- write(Codes) ,number_codes(I, Codes), !.
+joy_token(int(I), Codes) :- number_codes(I, Codes), !.
 joy_token(bool(true), "true") :- !.
 joy_token(bool(false), "false") :- !.
 joy_token(symbol(S), Codes) :- atom_codes(S, Codes).
@@ -67,3 +74,98 @@ blank --> [226, 128, 169].
 blank --> [226, 128, 175].
 blank --> [226, 129, 159].
 blank --> [227, 128, 128].
+
+
+thun([], S, S).
+thun([Term|E], Si, So) :- thun(Term, E, Si, So).
+
+thun(A,    [], S, [A|S]) :- var(A), !.
+thun(A, [T|E], S,   So)  :- var(A), !, thun(T, E, [A|S], So).
+
+thun(int(A),    [], B, [int(A)|B]).
+thun(int(C), [A|B], D, E) :- thun(A, B, [int(C)|D], E).
+
+thun(bool(A),    [], B, [bool(A)|B]).
+thun(bool(C), [A|B], D, E) :- thun(A, B, [bool(C)|D], E).
+
+thun(list(A),    [], B, [list(A)|B]).
+thun(list(C), [A|B], D, E) :- thun(A, B, [list(C)|D], E).
+
+thun(symbol(A),    [], B, C) :- func(A, B, C).
+thun(symbol(A), [C|D], B, F) :- func(A, B, E), thun(C, D, E, F).
+
+thun(symbol(Combo), E, Si, So) :- combo(Combo, Si, S, E, Eo), thun(Eo, S, So).
+
+thun(symbol(D),     [], Si, So) :- def(D, [DH| E]), thun(DH, E, Si, So).
+thun(symbol(D), [H|E0], Si, So) :- def(D, [DH|DE]),
+     append(DE, [H|E0], E), /* ................. */ thun(DH, E, Si, So).
+
+% Some error handling.
+
+thun(symbol(Unknown), _, _, _) :-
+    \+ def(Unknown, _),
+    \+ func(Unknown, _, _),
+    \+ combo(Unknown, _, _, _, _),
+    write('Unknown: '),
+    write(Unknown),
+    fail.
+
+
+func(swap, [A, B|S],  [B, A|S]).
+func(dup,     [A|S],  [A, A|S]).
+func(pop,     [_|S],        S ).
+
+func(cons,   [list(A),      B |S], [list([B|A])|S]).
+func(concat, [list(A), list(B)|S],     [list(C)|S]) :- append(B, A, C).
+
+func(swaack,    [list(R)|S],   [list(S)|R]).
+func(stack,              S ,   [list(S)|S]).
+func(clear,              _ ,            []).
+func(first, [list([X|_])|S],   [     X |S]).
+func(rest,  [list([_|X])|S],   [list(X)|S]).
+
+func(bool, [     int(0)|S], [bool(false)|S]).
+func(bool, [   list([])|S], [bool(false)|S]).
+func(bool, [bool(false)|S], [bool(false)|S]).
+
+func(bool, [     int(N)|S], [bool(true)|S]) :- N #\= 0.
+func(bool, [list([_|_])|S], [bool(true)|S]).
+func(bool, [ bool(true)|S], [bool(true)|S]).
+
+func( + ,  [int(A), int(B)|S], [int(A + B)|S]).
+func( - ,  [int(A), int(B)|S], [int(B - A)|S]).
+func( * ,  [int(A), int(B)|S], [int(A * B)|S]).
+func( / ,  [int(A), int(B)|S], [int(B div A)|S]).
+func('%',  [int(A), int(B)|S], [int(B mod A)|S]).
+
+func( add ,  [int(A), int(B)|S], [int(A + B)|S]).
+func( sub ,  [int(A), int(B)|S], [int(B - A)|S]).
+func( mul ,  [int(A), int(B)|S], [int(A * B)|S]).
+func( div ,  [int(A), int(B)|S], [int(B div A)|S]).
+func( mod,  [int(A), int(B)|S], [int(B mod A)|S]).
+
+
+combo(i,          [list(P)|S], S, Ei, Eo) :- append(P, Ei, Eo).
+combo(dip,     [list(P), X|S], S, Ei, Eo) :- append(P, [X|Ei], Eo).
+
+combo(branch, [list(T), list(_),  bool(true)|S], S, Ei, Eo) :- append(T, Ei, Eo).
+combo(branch, [list(_), list(F), bool(false)|S], S, Ei, Eo) :- append(F, Ei, Eo).
+
+combo(loop, [list(_), bool(false)|S], S, E,  E ).
+combo(loop, [list(B),  bool(true)|S], S, Ei, Eo) :- append(B, [list(B), symbol(loop)|Ei], Eo).
+
+
+joy_def(Codes) :-
+    text_to_expression(Codes, [symbol(Name)|Body]),
+    assert_def(Name, Body).
+
+assert_def(Symbol, Body) :-
+    (  % Don't let this "shadow" functions or combinators.
+        \+ func(Symbol, _, _),
+        \+ combo(Symbol, _, _, _, _)
+    ) -> (  % Replace any existing defs of this name.
+        retractall(def(Symbol, _)),
+        assertz(def(Symbol, Body))
+    ) ; true.
+
+joydef("enstacken stack [clear] dip").
\ No newline at end of file