(* ------------------------------------------------------------- *)

Lemma Par_EtaRel_exists : forall (G: context) D a b b' x,
x `notin` union (fv_tm_tm_tm a) (fv_tm_tm_tm b) ->
Par G D b b' ->
(open_tm_wrt_tm a (a_Var_f x)) = a_App b Rel (a_Var_f x) ->
Par G D (a_UAbs Rel a) b'.