(* Lemma headptr_field_compatible' {cs: compspecs}: forall t p, headptr p -> legal_alignas_type t = true -> legal_cosu_type t = true -> sizeof t < Ptrofs.modulus -> complete_type cenv_cs t = true -> field_compatible t nil p. *)

Lemma data_at_ext_derives {cs} sh t v v' p q: v=v' -> p=q -> @data_at cs sh t v p |-- @data_at cs sh t v' q.