Fixpoint be_iter1 (be : bool_expr) (n : nat) {struct n} : bool_expr :=
    match n with
    | O => be
    | S m => be_iter1 (bef be) m
    end.Fixpoint be_iter2 (be : bool_expr) (n : nat) {struct n} : bool_expr :=
    match n with
    | O => be
    | S m => bef (be_iter2 be m)
    end.

Lemma be_iter1_plus : forall (m n : nat) (be : bool_expr), be_iter1 (be_iter1 be m) n = be_iter1 be (m + n).