:- [pg].

article :-
	name(FileName, "article.pl"),
	open(FileName,write,Stream),
	sigma_pg(pg(f,[f,g],[
	     [(f(0) -> zero), (f(n+1) -> g(n+1)*f(n))],
	     [(g(0) -> zero),(g(n+1) -> suc(g(n)))]
	    ]),Stream),
	close(Stream).