proposition: Lemma cent1E x y : (x \in 'C[y]) = (x * y == y * x).
proof: Proof. by rewrite (sameP (cent1P x y) eqP). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)