proposition: Lemma gedge2 p : gedge (gedge p) = p.
proof: Proof. by rewrite {1}/gedge oddg_edge !arcg_ccw2 -opprD addrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)