Hint Rewrite mapsto_force_ptr: norm.
(******************************************  Definition of at_offset.  at_offset is the elementary definition. All useful lemmas about at_offset will be proved here.  ******************************************)
Definition at_offset (P: val -> mpred) (z: Z): val -> mpred :=
fun v => P (offset_val z v).
Arguments at_offset P z v : simpl never.

Lemma at_offset_eq3: forall P z b ofs,
at_offset P z (Vptr b (Ptrofs.repr ofs)) = P (Vptr b (Ptrofs.repr (ofs + z))).