Hint Rewrite wand_sepcon wand_sepcon' : norm.

Lemma extract_nth_exists_in_SEP:
forall n P Q (R: list mpred)
{A} (S: A -> mpred),
nth n R emp = (exp S) ->
PROPx P (LOCALx Q (SEPx R)) =
exp (fun x => PROPx P (LOCALx Q (SEPx (replace_nth n R (S x))))).