set(auto). 
set(print_lists_at_end). 
lex([c, h(_), g(_,_),f(_)]).
list(usable).
x = x.
f(g(h(x),x)) = c.
f(g(x,h(x))) = g(x,h(x)).
g(h(x),x) = g(y,h(y)).
end_of_list.