proposition: Lemma closed_Gd : fclosed node (codom hd).
proof: Proof. apply: (intro_closed cnodeC) => x _ /eqP <- xdP. by rewrite -(f_iinv xdP) -hdN codom_f. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry jordan color coloring)