+ *
+ * Warning: if we're in a loop, more than one update frame on
+ * the stack may point to the same object. Be careful not to
+ * overwrite an IND_OLDGEN in this case, because we'll screw
+ * up the mutable lists. To be on the safe side, don't
+ * overwrite any kind of indirection at all. See also
+ * threadSqueezeStack in GC.c, where we have to make a similar
+ * check.