Ltac cnf_simpl :=
        repeat match goal with
               | |- context[and_cnf_opt cnf_ff _ ] => rewrite and_cnf_opt_cnf_ff
               | |- context[and_cnf_opt _ cnf_ff] => rewrite and_cnf_opt_cnf_ff_r
               | |- context[and_cnf_opt _ cnf_tt] => rewrite and_cnf_opt_cnf_tt
               | |- context[or_cnf_opt cnf_ff _] => rewrite or_cnf_opt_cnf_ff
               | |- context[or_cnf_opt _ cnf_ff] => rewrite or_cnf_opt_cnf_ff_r
               end.

Lemma abst_iff_correct : forall (k: kind) (f1 f2 : GFormula k) (IHf1 : forall pol : bool, xcnf pol f1 = xcnf pol (abst_form pol f1)) (IHf2 : forall pol : bool, xcnf pol f2 = xcnf pol (abst_form pol f2)) (pol : bool), xcnf pol (IFF f1 f2) = xcnf pol (abst_iff abst_form pol f1 f2).