%:- op(1050, xfx, &).
%:- op(900, xfx, ==>).
:- op(800, xfx, &).
:- op(900, xfx, ==>).


cong(N, L) :-
	grammar(Rules),
	cong_(start, Rules, N, _, L, []).

cong_(T, Rules, N, N, [T|Ss], Ss) :- % terminal
	atom(T),
	\+ member(T ==> _, Rules),
	\+ member(T, Rules).
cong_(NT, Rules, N0, N, Ss0, Ss) :-
	member(NT ==> Body, Rules), % non-terminal
	N0 > 0,
	N1 is N0 - 1,
	cong_(Body, Rules, N1, N, Ss0, Ss).
cong_(NT, Rules, N0, N, Ss, Ss) :-
	member(NT, Rules), % non-terminal without body
	N0 > 0,
	N is N0 - 1.
cong_((A,B), Rules, N0, N, Ss0, Ss) :-
	cong_(A, Rules, N0, N1, Ss0, Ss1),
	cong_(B, Rules, N1, N, Ss1, Ss).
cong_(A & B, Rules, N0, N, Ss0, Ss) :-
	cong_(A, Rules, N0, N1, Ss0, Ss),
	cong_(B, Rules, N1, N, Ss0, Ss).