(* predicate a is closed under "edge" relation e *)
Definition closed {T: finType} (e: rel T) (a: pred T) :=
  forall x y : T, e x y -> a x -> a y.

Lemma clos_trans_1n_deriv_plus (s1 s2: clauseset) : Relation_Operators.clos_trans_1n clauseset (fun s s' : clauseset => trans s s') s1 s2 -> deriv_plus s1 s2.