proposition: Lemma codom_gcompr : codom gcompr =i predC (gcomp z).
proof: Proof. move=> x; apply/imageP/idP => [[[y z'Gy] _ ->] // | z'Gx]. by exists ((sub _ : _ -> gcomp_rem) z'Gx). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry jordan color coloring)