proposition: Fact addC : commutative add.
proof: Proof. by case=> x1 y1 [x2 y2]; congr Gpoint; apply: addrC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)