After Raymond's remark, I changed the Stackless bits to
two fixed bits, position 15 and 16. It is right, why should these
be elsewhere.
diff --git a/Include/object.h b/Include/object.h
index 06e19be..252e99c 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -488,9 +488,9 @@
/* Objects support garbage collection (see objimp.h) */
#define Py_TPFLAGS_HAVE_GC (1L<<14)
-/* These last two bits are preserved for Stackless Python */
+/* These two bits are preserved for Stackless Python, next after this is 16 */
#ifdef STACKLESS
-#define Py_TPFLAGS_HAVE_STACKLESS_EXTENSION (3L<<(sizeof(int)*8-2))
+#define Py_TPFLAGS_HAVE_STACKLESS_EXTENSION (3L<<15)
#else
#define Py_TPFLAGS_HAVE_STACKLESS_EXTENSION 0
#endif