(* -----  exists forms of the binding constructors ----------- *)

Axiom An_CPi_exists :  ∀ c (G : context) (phi : constraint) (B : tm),
c \notin dom G \u fv_co_co_tm B ->
AnnPropWff G phi
→ AnnTyping ([(c, Co phi)] ++ G) (open_tm_wrt_co B (g_Var_f c)) a_Star
→ AnnTyping G (a_CPi phi B) a_Star.