proposition: Fact fmod_addrC : commutative fmod_add.
proof: Proof. by case=> x Ax [y Ay]; apply: val_inj; apply: (centsP abelA). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div choice fintype bigop ssralg finset fingroup) mathcomp(morphism perm finalg action gproduct commutator) mathcomp(cyclic)