fact(Fact):-
	clause(Fact, true).

kind_of(X, Y):-
	is_a(X, Thing),
	kind_of(Thing, Y).

kind_of(X, Y):-
	\+ kind(X),
	fact(kind_of(X, Z)),
	kind_of(Z, Y).

can_be(X, Y):-
	never(X, Y),
	!, fail.

can_be(X, Y):-
	is_a(X, Thing),
	can_be(Thing, Y).

can_be(X, Y):-
	can_be(X, Y, _).

can_be(X, Y, []):-
	fact(can_be(X, Y)).

can_be(X, Y, [Op|Ops]):-
	findall(Kind, kind_of(X, Kind), Kinds),
	member(K, [X|Kinds]),
	fact(can_be(K, [Op|Ops])),
	member(Y, [Op|Ops]),
	\+ never(X, Y),
	\+ (member(Always, [Op|Ops]), always(X, Always), Always \= Y).