:- [pg_with_trace].

fgh :-
	name(FileName, "ex_fgh.pl"),
	open(FileName,write,Stream),
	sigma_pg(pg(f,[f,g,h],[
	     [(f(0,v,w) -> g(v,w)), (f(s(u),v,w) -> f(u,s(v),w)+(f(u,s(v),w)+f(u,s(v),w)))],
	     [(g(0,w) -> h(w)),(g(s(v),w) -> g(v,w)*g(v,w))],
	     [(h(0) -> a),(h(s(w)) -> b(h(w)))]
	    ]),Stream),
	close(Stream).

% bash-2.03$ sicstus 
% SICStus 3.8.3 (sparc-solaris-5.7): Mon May  8 14:25:09 MET DST 2000
% Licensed to info.univ-angers.fr
% | ?- ['example_fgh.pg'].
% {consulting /home/info/helios/stephan/public_html/Research/GRAMMAIRES/PG/PROGRAMMATION/SICSTUS/example_fgh.pg...}
% {consulting /home/info/helios/stephan/public_html/Research/GRAMMAIRES/PG/PROGRAMMATION/SICSTUS/pg_with_trace.pl...}
% {loading /opt/SICStus-3.8.3/lib/sicstus-3.8.3/library/lists.po...}
% {module lists imported into user}
% {loaded /opt/SICStus-3.8.3/lib/sicstus-3.8.3/library/lists.po in module lists, 0 msec 20472 bytes}
% {consulted /home/info/helios/stephan/public_html/Research/GRAMMAIRES/PG/PROGRAMMATION/SICSTUS/pg_with_trace.pl in module user, 20 msec 36744 bytes}
% {consulted /home/info/helios/stephan/public_html/Research/GRAMMAIRES/PG/PROGRAMMATION/SICSTUS/example_fgh.pg in module user, 20 msec 37944 bytes}

% yes
% | ?- fgh.

% yes
% | ?- [ex_fgh].
% {consulting /home/info/helios/stephan/public_html/Research/GRAMMAIRES/PG/PROGRAMMATION/SICSTUS/ex_fgh.pl...}
% {consulted /home/info/helios/stephan/public_html/Research/GRAMMAIRES/PG/PROGRAMMATION/SICSTUS/ex_fgh.pl in module user, 0 msec 1816 bytes}

% yes
% | ?- phrase_pg(X,f,[s(0),s(0),s(0)],user_output).
% f+(f+f)
% g+(g+g)
% g*g+(g*g+g*g)
% g*g*(g*g)+(g*g*(g*g)+g*g*(g*g))
% h*h*(h*h)+(h*h*(h*h)+h*h*(h*h))
% b(h)*b(h)*(b(h)*b(h))+(b(h)*b(h)*(b(h)*b(h))+b(h)*b(h)*(b(h)*b(h)))
% b(a)*b(a)*(b(a)*b(a))+(b(a)*b(a)*(b(a)*b(a))+b(a)*b(a)*(b(a)*b(a)))

% X = b(a)*b(a)*(b(a)*b(a))+(b(a)*b(a)*(b(a)*b(a))+b(a)*b(a)*(b(a)*b(a))) ? 

% yes
% | ?-