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Definition of at_offset.

at_offset is the elementary definition. All useful lemmas about at_offset will be proved here.

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Definition at_offset (P: val -> mpred) (z: Z): val -> mpred :=
 fun v => P (offset_val z v).

Lemma at_offset_eq2: forall pos pos' P, forall p, at_offset P (pos + pos') p = at_offset P pos' (offset_val pos p).