% Harry Potter and the seven potions

perm([],[]).
perm(X,[Y|Z]):-select(X,Y,R),perm(R,Z).

puzzle(Potions, Dwarf, Giant):- Potions=[P1,P2,P3,P4,P5,P6,P7],
	F=forward, B=back, W=wine, P=poison,
	perm(Potions, [F,B,W,W,P,P,P]),
	P1\==W,
	(P2\==W; P1=P),
	(P3\==W; P2=P),
	(P4\==W; P3=P),
	(P5\==W; P4=P),
	(P6\==W; P5=P),
	(P7\==W; P6=P),
	P1\==P7,
	P1\==F,
	P7\==F,
	P2==P6,
	select(Potions, Dwarf, BigPotions), member(Giant, BigPotions),
	Dwarf\==P,
	Giant\==P,
	P7=back,
	Dwarf=forward.
solve(S):-setof([P,D,G],puzzle(P,D,G),S),member(T,S), write(T),nl,fail.

:-print('solve(X), puzzle(X,Y,Z), ... loaded').
:-nl.