proposition: Lemma halfg_eq p c : is_oddg c -> halfg (p *+ 2 + c) = p.
proof: Proof. by rewrite halfg_add2 -[RHS]addr0 => /halfg_odd->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)