Fixpoint relative_defined_type {A: Type} (l: list (ident * A)) (t: type): Prop :=
  match t with
  | Tarray t' _ _ => relative_defined_type l t'
  | Tstruct id _ => In id (map fst l)
  | Tunion id _ => In id (map fst l)
  | _ => True
  end.

Lemma relative_defined_type_mono: forall {A B: Type} (l1: list (ident * A)) (l2: list (ident * B)) (t: type), (forall i, In i (map fst l1) -> In i (map fst l2)) -> relative_defined_type l1 t -> relative_defined_type l2 t.