proposition: Lemma ex4_system_prop: projective_system_on ex4_system (Lg I ex4_setEi) I r ex4_function_f_fam.
proof: Proof. by rewrite /ex4_system; case: ex4H => [pa pb pc pd pe]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)