Prove another lemma to open invariants that we only know by namespace.

The good news is, this one works without FSAs, and it can be applied around the "view shift with a step"-thing.
Furthermore, the FSA lemma can be derived from the new one.
The bad news is, the FSA lemma proof doesn't even get shorter in doing this change.

program_logic/invariants.v

 From iris.program_logic Require Import ownership.
-From iris.program_logic Require Export namespaces.
+From iris.program_logic Require Export namespaces lviewshifts.
 From iris.proofmode Require Import pviewshifts.
 Import uPred.
 
@@ -31,16 +31,34 @@ Proof.
   by rewrite /inv (pvs_allocI N); last apply coPset_suffixes_infinite.
 Qed.
 
-(** Invariants can be opened around any frame-shifting assertion. *)
+(** Fairly explicit form of opening invariants *)
+Lemma inv_open E N P :
+  nclose N ⊆ E →
+  inv N P ⊢ ∃ E', ■ (E ∖ nclose N ⊆ E' ∧ E' ⊆ E) ★
+                    |={E,E'}=> ▷ P ★ (▷ P ={E',E}=★ True).
+Proof.
+  rewrite /inv. iIntros {?} "Hinv". iDestruct "Hinv" as {i} "[% #Hi]".
+  iExists (E ∖ {[ i ]}). iSplit. { iPureIntro. set_solver. }
+  iPvs (pvs_openI' with "Hi") as "HP"; [set_solver..|].
+  iPvsIntro. iSplitL "HP"; first done. iIntros "HP".
+  iPvs (pvs_closeI' _ _ P with "[HP]"); [set_solver|iSplit; done|set_solver|].
+  iPvsIntro. done.
+Qed.
+
+(** Invariants can be opened around any frame-shifting assertion. This is less
+    verbose to apply than [inv_open]. *)
 Lemma inv_fsa {A} (fsa : FSA Λ Σ A) `{!FrameShiftAssertion fsaV fsa} E N P Ψ :
   fsaV → nclose N ⊆ E →
   (inv N P ★ (▷ P -★ fsa (E ∖ nclose N) (λ a, ▷ P ★ Ψ a))) ⊢ fsa E Ψ.
 Proof.
-  iIntros {??} "[#Hinv HΨ]"; rewrite /inv; iDestruct "Hinv" as {i} "[% Hi]".
-  iApply (fsa_open_close E (E ∖ {[encode i]})); auto; first by set_solver.
-  iPvs (pvs_openI' _ _ with "Hi") as "HP"; [set_solver..|iPvsIntro].
-  iApply (fsa_mask_weaken _ (E ∖ N)); first set_solver.
-  iApply fsa_wand_r; iSplitL; [by iApply "HΨ"|iIntros {v} "[HP HΨ]"].
-  iPvs (pvs_closeI' _ _ P with "[HP]"); [auto|by iSplit|set_solver|done].
+  iIntros {??} "[Hinv HΨ]".
+  iDestruct (inv_open E N P with "Hinv") as {E'} "[% Hvs]"; first done.
+  iApply (fsa_open_close E E'); auto; first set_solver.
+  iPvs "Hvs" as "[HP Hvs]";first set_solver.
+  (* TODO: How do I do sth. like [iSpecialize "HΨ HP"]? *)
+  iPvsIntro. iApply (fsa_mask_weaken _ (E ∖ N)); first set_solver.
+  iApply fsa_wand_r. iSplitR "Hvs"; first by iApply "HΨ".
+  simpl. iIntros {v} "[HP HΨ]". iPvs ("Hvs" with "HP"); first set_solver.
+  by iPvsIntro.
 Qed.
 End inv.