Proof with simpl; auto.
induction l...
destruct a... destruct gamma... destruct delta...
destruct p... destruct delta...
intros [H1 H2]. simpl in H1. spec H1. auto.
inversion H1 as [H3|H3]; [|inversion H3].
apply expr_merge_sound...
Qed.

Lemma cclose_sound l s :
listd clause_denote inter TT l s -> listd clause_denote inter TT (cclose l) s.