+ This is the write barrier for MUT_VARs, a.k.a. IORefs. A
+ MUT_VAR_CLEAN object is not on the mutable list; a MUT_VAR_DIRTY
+ is. When written to, a MUT_VAR_CLEAN turns into a MUT_VAR_DIRTY
+ and is put on the mutable list.
+ -------------------------------------------------------------------------- */
+
+void dirty_MUT_VAR(StgClosure *p);
+
+/* -----------------------------------------------------------------------------