Proof with auto.
intros H1 H2. unfold ef. destruct ce... destruct c... destruct delta...
destruct p... destruct e... if_tac...
apply ef_aux_sound... simpl; auto. apply var_eq_refl.
Qed.

Lemma delete_resolved_sound c s :
clause_denote c s -> clause_denote (delete_resolved c) s.