proposition: Fact addA : associative add.
proof: Proof. by case=> x1 y1 [x2 y2] [x3 y3] /=; rewrite !addrA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)