Fixpoint arities (e:BC) : Arities :=
  match e with
  | zero => ok_arities 0 0
  | proj n s i => 
    if leb (S i) (n + s) then ok_arities n s
      else error_proj n s i
  | succ _ => ok_arities 0 1
  | pred => ok_arities 0 1
  | cond => ok_arities 0 4
  | rec g h0 h1 =>
      match arities g, arities h0, arities h1 
        with | ok_arities gn gs, ok_arities h0n h0s, ok_arities h1n h1s =>
          if beq_nat (S gn) h0n && beq_nat h0n h1n &&
            beq_nat (S gs) h0s && beq_nat h0s h1s 
          then ok_arities h0n gs 
            else error_rec (ok_arities gn gs) (ok_arities h0n h0s) (ok_arities h1n h1s)
        | e1, e2, e3 => error_rec e1 e2 e3
      end
    | comp n s h nl sl =>
      match arities h, map arities nl, map arities sl with
      | ok_arities hn hs, anl, asl =>
        if beq_nat hn (length nl) && beq_nat hs (length sl) && 
          forallb (fun ne => aeq (arities ne) (ok_arities n 0)) nl &&
          forallb (fun se => aeq (arities se) (ok_arities n s)) sl
          then ok_arities n s
          else error_comp (ok_arities hn hs) anl asl 
      | e , anl, asl => error_comp e anl asl
      end
  end.Definition arities2 e :=
  match arities e with
    | ok_arities n s => (n, s)
    | _ => (0, 0)
  end.

Lemma arities_nth : forall l i e n s, (forall e, In e l -> arities e = ok_arities n s) -> arities e = ok_arities n s -> arities (nth i l e) = ok_arities n s.