Useful comment added.
(this is really a CVS checkin test, please ignore...) --keithw
This is also used in @LambdaFormInfo@ in the @ClosureInfo@ module.
+A @ReEntrant@ closure may be entered multiple times, but should not be
+updated or blackholed. An @Updatable@ closure should be updated after
+evaluation (and may be blackholed during evaluation). A @SingleEntry@
+closure will only be entered once, and so need not be updated but may
+safely be blackholed.
+
\begin{code}
data UpdateFlag = ReEntrant | Updatable | SingleEntry