From a bug report:
> If the Summon Nasties monster spell gates in two minions instead of one,
> the message still says "A monster appears from nowhere!"
The code wasn't counting any summoned monsters who had an opposite alignment
to the summoner. It also assumed that the 10% chance for demon summoning
in Gehennom always yielded exactly one monster even though that can produce
zero or more than one.
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