Proof with simpl; auto; try solve[congruence].
induction l...
remember (rev_cmp compare_clause c' a) as b; destruct b...
inversion 1; subst... cut (c' = c \/ In c l)... inversion 1...
Qed.

(* Specifications: *)
Lemma compare_clause_refl d : Eq = rev_cmp compare_clause d d.
Proof with simpl; auto.