(** * Length-bounding condition that must hold for a Cobham expression to be valid *)

Fixpoint rec_bounded' (e : Cobham) : Prop :=
  match e with
    | Rec g h0 h1 j =>
      rec_bounded' j  /\ 
      rec_bounded' g /\ 
      rec_bounded' h0 /\ 
      rec_bounded' h1 /\
      (match (arity e) with
         | ok_arity n => forall l, length l = n ->
           length (Sem e l) <= length (Sem j l)
        | _ => True (* never happen *)
       end)
    | Comp n h l => rec_bounded' h /\
        andl rec_bounded' l
    | _ => True
  end.Fixpoint rec_bounded (e : Cobham) : Prop :=
  match e with
    | Rec g h0 h1 j =>
      rec_bounded j  /\ 
      rec_bounded g /\ 
      rec_bounded h0 /\ 
      rec_bounded h1 /\
      (forall l, 
        length (Sem e l) <= length (Sem j l))
    | Comp n h l => rec_bounded h /\
        andl rec_bounded l
    | _ => True
  end.

Lemma rec_bounded'_spec : forall (e : Cobham) n, arity e = ok_arity n -> rec_bounded' e -> rec_bounded e.