proposition: Lemma oddg_eq p c : is_oddg c -> oddg (p *+ 2 + c) = c.
proof: Proof. by rewrite oddg_add2 => /oddg_id. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)