add FIXMEs for Coq bug work-arounds

theories/hashset.v

@@ -104,6 +104,7 @@ Proof.
     intros (l&?&?). exists (hash x, l); simpl. by rewrite elem_of_map_to_list.
   - unfold elements, hashset_elements. intros [m Hm]; simpl.
     rewrite map_Forall_to_list in Hm. generalize (NoDup_fst_map_to_list m).
+    (* FIXME: trailing [?] works around Coq bug #12944. *)
     induction Hm as [|[n l] m' [??] Hm ?];
       csimpl; inversion_clear 1 as [|?? Hn]; [constructor|].
     apply NoDup_app; split_and?; eauto.

theories/sorting.v

@@ -180,6 +180,7 @@ Section merge_sort_correct.
     Sorted R l1 → Sorted R l2 → Sorted R (list_merge R l1 l2).
   Proof.
     intros Hl1. revert l2. induction Hl1 as [|x1 l1 IH1];
+      (* FIXME: trailing [?] works around Coq bug #12944. *)
       induction 1 as [|x2 l2 IH2 ?]; rewrite ?list_merge_cons; simpl;
       repeat case_decide;
       repeat match goal with H : ¬R _ _ |- _ => apply total_not in H end;