+-- ToDo (SDM, 7 Jan 2003): I'm not sure that the distinction between
+-- Free and NonPointer in the free list is needed any more. It used
+-- to be needed because we constructed bitmaps from the free list, but
+-- now we construct bitmaps by finding all the live pointer bindings
+-- instead. Non-pointer stack slots (i.e. saved cost centres) can
+-- just be removed from the free list instead of being recorded as a
+-- NonPointer.
+