proposition: Lemma codom_embr : codom embr =i [predC rdom'].
proof: Proof. move=> x; apply/imageP/idP=> [[w _ ->] | bG'x]; first exact: (valP w). by exists (sub x bG'x). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color chromogram coloring patch) fourcolor(snip revsnip kempe birkhoff contract)