proposition: Lemma oddg_double p : oddg (p *+ 2) = 0.
proof: Proof. by rewrite -[p *+ 2]addr0 oddg_add2. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)