proposition: Lemma ccw4 p : ccw (ccw (ccw (ccw p))) = p.
proof: Proof. by case: p => x y; congr (Gpoint _ _); apply: subKr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)