fix the proof fro sum_frac_all correctness

theories/vcgen/vcgen.v

@@ -217,6 +217,7 @@ Instance dval_EqDecision : EqDecision dval.
     iFrame. iApply ("H" with "Hps").
   Qed.
 
+
   Lemma sum_frac_correct E p ps:
     env_ps_dv_interp E (sum_frac ps p) ⊣⊢ env_ps_dv_interp E (p :: ps).
   Proof.
@@ -252,9 +253,14 @@ Instance dval_EqDecision : EqDecision dval.
     - done.
     - intros acc. simpl. specialize (IHps  ((sum_frac acc a))). rewrite -IHps.
       rewrite -env_ps_dv_interp_app. rewrite sum_frac_correct.
-      assert (a :: ps ++ acc = ps ++ a :: acc). admit.
-      rewrite H0. by rewrite !env_ps_dv_interp_app.
-  Admitted.
+      rewrite !app_comm_cons.
+      rewrite (app_comm_cons [] acc).
+      rewrite (app_comm_cons [] ps).
+      rewrite -!env_ps_dv_interp_app.
+      iStartProof. iSplit.
+      + iIntros "[[H1  H2] H3]"; eauto with iFrame.
+      + iIntros "(H1 & H2 & H3)". eauto with iFrame.
+  Qed.
 
   Lemma sum_frac_all_correct E ps :
     env_ps_dv_interp E ps ⊣⊢ env_ps_dv_interp E (sum_frac_all ps).