(* Calls forward_if with the current precondition to which the provided conditions are added *)
(* QQQ TODO does this already exist? Add to library? *)
Ltac forward_if_diff add := match add with
| (PROPx ?P2 (LOCALx ?Q2 (SEPx ?R2))) => match goal with
| |- semax ?Delta (PROPx ?P1 (LOCALx ?Q1 (SEPx ?R1))) _ _ =>
let P3 := fresh "P3" in let Q3 := fresh "Q3" in let R3 := fresh "R3" in
pose (P3 := P1 ++ P2); pose (Q3 := Q1 ++ Q2); pose (R3 := R1 ++ R2);
simpl in P3, Q3, R3;
forward_if (PROPx P3 (LOCALx Q3 (SEPx R3)));
subst P3 Q3 R3
end
end.

(* TODO floyd this lemma should be invoked by entailer! Note: in summaray, this already works for data_at, but why doesn't it work for field_at? QQQ: And why/how does it work for data_at? *)
Lemma field_at_update_val: forall sh t gfs v v' p,
v = v' -> field_at sh t gfs v p |-- field_at sh t gfs v' p.