+ tso->blocked_exceptions is either:
+
+ NULL if async exceptions are unblocked.
+
+ END_TSO_QUEUE if async exceptions are blocked, but no threads
+ are currently waiting to deliver.
+
+ (StgTSO *)tso if threads are currently awaiting delivery of
+ exceptions to this thread.
+
+ The 2 cases BlockedOnGA and BlockedOnGA_NoSend are needed in a GUM
+ setup only. They mark a TSO that has entered a FETCH_ME or
+ FETCH_ME_BQ closure, respectively; only the first TSO hitting the
+ closure will send a Fetch message.
+ Currently we have no separate code for blocking on an RBH; we use the
+ BlockedOnBlackHole case for that. -- HWL
+
+ ---------------------------------------------------------------------------- */