Ltac go_lowerx' simpl_tac :=
   unfold PROPx, LOCALx,SEPx, local, lift1; unfold_lift; intro rho; simpl_tac;
   repeat rewrite andp_assoc;
   repeat ((simple apply go_lower_lem1 || apply derives_extract_prop || apply derives_extract_prop'); intro);
   try apply prop_left;
   repeat rewrite prop_true_andp by assumption;
   try apply derives_refl.Ltac go_lowerx := go_lowerx' simpl.Ltac go_lowerx_no_simpl := go_lowerx' idtac.

Lemma grab_nth_SEP: forall n P Q R, PROPx P (LOCALx Q (SEPx R)) = (PROPx P (LOCALx Q (SEPx (nth n R emp :: delete_nth n R)))).