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

grammar([start ==> xa & (xb & xc),
	    xa ==> (a, xa),
	    xa,
	    xb ==> (a, a, xb),
	    xb,
	    xc ==> (a, a, a, xc),
	    xc
	   ]).

/*

the order (and multiplicity) of the answers is not important - but
execution of these queries should be finite

?- cong(10,L), write(L), nl, fail.
[]

?- cong(20,L), write(L), nl, fail.
[a,a,a,a,a,a]
[]

?- cong(30,L), write(L), nl, fail.
[a,a,a,a,a,a,a,a,a,a,a,a]
[a,a,a,a,a,a]
[]

*/