Ltac extract_exists_in_SEP :=
match goal with |- @semax _ _ _ (PROPx _ (LOCALx _ (SEPx ?R))) _ _ =>
match R with context [ exp ?z :: _] =>
let n := find_in_list (exp z) R
in rewrite (grab_nth_SEP n); unfold nth, delete_nth; rewrite extract_exists_in_SEP;
repeat_extract_exists_pre
end
end.

Lemma flatten_sepcon_in_SEP'':
forall n P Q (R1 R2: mpred) (R: list mpred) R',
nth_error R n = Some ((R1 * R2)) ->
R' = firstn n R ++ R1 :: R2 :: skipn (S n) R ->
PROPx P (LOCALx Q (SEPx R)) = PROPx P (LOCALx Q (SEPx R')).