diff --git a/mps/code/arena.c b/mps/code/arena.c
index 5101908875b..12d7af1137b 100644
--- a/mps/code/arena.c
+++ b/mps/code/arena.c
@@ -388,6 +388,7 @@ void ArenaDestroy(Arena arena)
   LandFinish(ArenaFreeLand(arena));
 
   /* The CBS block pool can't free its own memory via ArenaFree because
+     that would use the ZonedCBS. */
   MFSFinishTracts(ArenaCBSBlockPool(arena), arenaMFSPageFreeVisitor,
                   UNUSED_POINTER, UNUSED_SIZE);
 
diff --git a/mps/code/arenacl.c b/mps/code/arenacl.c
index 58d1abce445..cb4d9865ae3 100644
--- a/mps/code/arenacl.c
+++ b/mps/code/arenacl.c
@@ -179,9 +179,9 @@ static Bool clientChunkDestroy(Tree tree, void *closureP, Size closureS)
   ClientChunk clChunk;
 
   AVERT(Tree, tree);
-  /* FIXME: AVER(closureP == UNUSED_POINTER); */
+  AVER(closureP == UNUSED_POINTER);
   UNUSED(closureP);
-  /* FIXME: AVER(closureS == UNUSED_SIZE); */
+  AVER(closureS == UNUSED_SIZE);
   UNUSED(closureS);
   
   chunk = ChunkOfTree(tree);
@@ -308,8 +308,8 @@ static void ClientArenaFinish(Arena arena)
   /* Destroy all chunks, including the primary. See
    * <design/arena/#chunk.delete> */
   arena->primary = NULL;
-  /* FIXME: use UNUSED_POINTER, UNUSED_SIZE instead of NULL, 0 */
-  TreeTraverseAndDelete(&arena->chunkTree, clientChunkDestroy, NULL, 0);
+  TreeTraverseAndDelete(&arena->chunkTree, clientChunkDestroy,
+                        UNUSED_POINTER, UNUSED_SIZE);
 
   clientArena->sig = SigInvalid;
 
diff --git a/mps/code/arenavm.c b/mps/code/arenavm.c
index 800e7aeca21..73ea711f39c 100644
--- a/mps/code/arenavm.c
+++ b/mps/code/arenavm.c
@@ -408,9 +408,9 @@ static Bool vmChunkDestroy(Tree tree, void *closureP, Size closureS)
   VMChunk vmChunk;
 
   AVERT(Tree, tree);
-  /* FIXME: AVER(closureP == UNUSED_POINTER); */
+  AVER(closureP == UNUSED_POINTER);
   UNUSED(closureP);
-  /* FIXME: AVER(closureS == UNUSED_SIZE); */
+  AVER(closureS == UNUSED_SIZE);
   UNUSED(closureS);
 
   chunk = ChunkOfTree(tree);
@@ -609,8 +609,8 @@ static void VMArenaFinish(Arena arena)
   /* Destroy all chunks, including the primary. See
    * <design/arena/#chunk.delete> */
   arena->primary = NULL;
-  /* FIXME: use UNUSED_POINTER, UNUSED_SIZE instead of NULL, 0 */
-  TreeTraverseAndDelete(&arena->chunkTree, vmChunkDestroy, NULL, 0);
+  TreeTraverseAndDelete(&arena->chunkTree, vmChunkDestroy,
+                        UNUSED_POINTER, UNUSED_SIZE);
   
   /* Destroying the chunks should have purged and removed all spare pages. */
   RingFinish(&vmArena->spareRing);
@@ -1102,7 +1102,7 @@ static Bool vmChunkCompact(Tree tree, void *closureP, Size closureS)
 
   AVERT(Tree, tree);
   AVERT(Arena, arena);
-  /* FIXME: AVER(closureS == UNUSED_SIZE); */
+  AVER(closureS == UNUSED_SIZE);
   UNUSED(closureS);
 
   vmArena = Arena2VMArena(arena);
@@ -1114,7 +1114,7 @@ static Bool vmChunkCompact(Tree tree, void *closureP, Size closureS)
   {
     Addr base = chunk->base;
     Size size = ChunkSize(chunk);
-    vmChunkDestroy(tree, closureP, closureS);
+    vmChunkDestroy(tree, UNUSED_POINTER, UNUSED_SIZE);
     vmArena->contracted(arena, base, size);
     return TRUE;
   } else {
@@ -1138,8 +1138,8 @@ static void VMCompact(Arena arena, Trace trace)
   /* Destroy chunks that are completely free, but not the primary
    * chunk. See <design/arena/#chunk.delete>
    * TODO: add hysteresis here. See job003815. */
-  /* FIXME: use UNUSED_SIZE instead of 0 */
-  TreeTraverseAndDelete(&arena->chunkTree, vmChunkCompact, arena, 0);
+  TreeTraverseAndDelete(&arena->chunkTree, vmChunkCompact, arena,
+                        UNUSED_SIZE);
 
   {
     Size vmem0 = trace->preTraceArenaReserved;