proposition: Lemma arcg_ccw2 c : arcg (ccw (ccw c)) = - arcg c.
proof: Proof. by rewrite /arcg !ccw2 !opprB addrCA addrAC add0r. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)