Add more Prolog samples

Closes #233

samples/Prolog/normal_form.pl

@@ -0,0 +1,94 @@
+%%----- normalize(+Wff,-NormalClauses) ------
+normalize(Wff,NormalClauses) :-
+   conVert(Wff,[],S),
+   cnF(S,T),
+   flatten_and(T,U),
+   make_clauses(U,NormalClauses).
+
+%%-----  make a sequence out of a conjunction -----
+flatten_and(X /\ Y, F) :-
+   !,
+   flatten_and(X,A),
+   flatten_and(Y, B),
+   sequence_append(A,B,F).
+flatten_and(X,X).
+
+%%-----  make a sequence out of a disjunction -----
+flatten_or(X \/ Y, F) :-
+   !,
+   flatten_or(X,A),
+   flatten_or(Y,B),
+   sequence_append(A,B,F).
+flatten_or(X,X).
+
+
+%%----- append two sequences -------------------------------
+sequence_append((X,R),S,(X,T)) :- !, sequence_append(R,S,T).
+sequence_append((X),S,(X,S)).
+
+%%----- separate into positive and negative literals -----------
+separate((A,B),P,N) :-
+   !,
+   (A = ~X -> N=[X|N1],
+               separate(B,P,N1)
+             ;
+               P=[A|P1],
+               separate(B,P1,N) ).
+separate(A,P,N) :-
+   (A = ~X -> N=[X],
+               P = []
+            ;
+               P=[A],
+               N = [] ).
+
+%%----- tautology ----------------------------
+tautology(P,N) :- some_occurs(N,P).
+
+some_occurs([F|R],B) :-
+   occurs(F,B) | some_occurs(R,B).
+
+occurs(A,[F|_]) :-
+   A == F,
+   !.
+occurs(A,[_|R]) :-
+   occurs(A,R).
+
+make_clauses((A,B),C) :-
+   !,
+   flatten_or(A,F),
+   separate(F,P,N),
+   (tautology(P,N) ->
+      make_clauses(B,C)
+          ;
+      make_clause(P,N,D),
+      C = [D|R],
+      make_clauses(B,R) ).
+make_clauses(A,C) :-
+   flatten_or(A,F),
+   separate(F,P,N),
+   (tautology(P,N) ->
+       C = []
+        ;
+       make_clause(P,N,D),
+       C = [D] ).
+
+make_clause([],N, false :- B) :-
+   !,
+   make_sequence(N,B,',').
+make_clause(P,[],H) :-
+   !,
+   make_sequence(P,H,'|').
+make_clause(P,N, H :- T) :-
+   make_sequence(P,H,'|'),
+   make_sequence(N,T,',').
+
+make_sequence([A],A,_) :- !.
+make_sequence([F|R],(F|S),'|') :-
+   make_sequence(R,S,'|').
+make_sequence([F|R],(F,S),',') :-
+   make_sequence(R,S,',').
+
+write_list([F|R]) :-
+   write(F), write('.'), nl,
+   write_list(R).
+write_list([]).