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