Definition eval_opt_clause (env : Env) (cl: option clause) :=
    match cl with
    | None => True
    | Some cl => eval_clause env cl
    end.

Lemma eval_cnf_cons_iff : forall env a f, ((~ make_conj (eval_tt env) a) /\ eval_cnf env f) <-> eval_cnf env (a::f).