When framing, `▷ emp` should be turned into `emp` if the BI is affine.

If the BI is not affine, this should not happen, as it may lead to
information loss.

This commit fixes issue #190.

tests/proofmode.v

@@ -158,6 +158,9 @@ Lemma test_iFrame_conjunction_2 P Q :
   P -∗ Q -∗ (P ∧ P) ∗ (Q ∧ Q).
 Proof. iIntros "HP HQ". iFrame "HP HQ". Qed.
 
+Lemma test_iFrame_later `{BiAffine PROP} P Q : P -∗ Q -∗ ▷ P ∗ Q.
+Proof. iIntros "H1 H2". by iFrame "H1". Qed.
+
 Lemma test_iAssert_modality P : ◇ False -∗ ▷ P.
 Proof.
   iIntros "HF".

theories/proofmode/frame_instances.v

@@ -270,6 +270,9 @@ Proof. rewrite /Frame /MakeEmbed -embed_internal_eq. apply (frame_embed p P Q).
 
 Global Instance make_laterN_true n : @KnownMakeLaterN PROP n True True | 0.
 Proof. by rewrite /KnownMakeLaterN /MakeLaterN laterN_True. Qed.
+Global Instance make_laterN_emp `{!BiAffine PROP} n :
+  @KnownMakeLaterN PROP n emp emp | 0.
+Proof. by rewrite /KnownMakeLaterN /MakeLaterN laterN_emp. Qed.
 Global Instance make_laterN_0 P : MakeLaterN 0 P P | 0.
 Proof. by rewrite /MakeLaterN. Qed.
 Global Instance make_laterN_1 P : MakeLaterN 1 P (▷ P) | 2.