Ltac gather_SEP' L :=
   grab_indexes_SEP L;
 match goal with |- context [SEPx ?R] =>
    let r := fresh "R" in
    set (r := (SEPx R));
    revert r;
     rewrite <- (firstn_skipn (length L) R);
      unfold length at 1 2;
      unfold firstn at 1; unfold skipn at 1;
      rewrite gather_SEP;
   unfold fold_right at 1; try  rewrite sepcon_emp;
   try (intro r; unfold r; clear r)
 end.Fixpoint replace_nth {A} (n: nat) (al: list A) (x: A) {struct n}: list A :=
 match n, al with
 | O , a::al => x::al
 | S n', a::al' => a :: replace_nth n' al' x
 | _, nil => nil
 end.Fixpoint my_nth{A} (n: nat) (al: list A) (default: A) {struct n} : A :=
  (* just like nth; make a new copy, for better control of unfolding *)
match n, al with
| O, a::al' => a
| S n', a::al' => my_nth n' al' default
| _, nil => default
end.

Lemma nth_error_replace_nth: forall {A:Type} R n (Rn Rn':A), nth_error R n = Some Rn -> nth_error (replace_nth n R Rn') n = Some Rn'.