rewrite/almost_full; move=> [p] H1; exists p.
apply: (@sec_strengthen _ _ (fun x y => eq_fin (cs2id x) (cs2id y)))=> // x y.
rewrite/cs2id=> /=; case=> H2; apply/eqP.
by apply: enum_rank_inj; apply: (ord_inj H2).
Qed.

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.