proposition: Lemma modz2 x : (x %% 2)%Z = odd `|x|.
proof: Proof. by rewrite -modn2 -modz_nat abszEsign -modzMml -modzXm expr1n mul1r. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)