The crawl arena is a special mode during which the player is absent and
monsters fight each other, useful for both testing/debugging and for
entertainment.  It can be invoked on the command line like this:
    crawl -arena "kobold v goblin"
Which will cause one kobold to fight one goblin, with crawl shutting down once
one of them has won.  To make them fight for three rounds you can do:
    crawl -arena "t:3 kobold v goblin"
You can make monsters fight for at most 99 rounds.  You can stop the
arena simulation early by pressing Escape, 'q' or Control-G (though if
the arena has lots of monsters it might take a few second before it
stops).
You can also give each side more than one monster.  For example:
    crawl -arena "rat, giant cockroach v kobold, goblin"
will give team A a rat and a cockroach and team B a kobold and a goblin; a
victory is declared when all of one team is dead and at lest one member of the
other team is still alive.
You can also make multiple of a single type of monster (note that the singular
form of the names are still used):
    crawl -arena "2 kobold v 3 goblin:
    crawl -arena "2 rat, 3 giant cockroach v 4 kobold, 5 goblin"
You can even have multiple instances of uniques:
    crawl -arena "10 Murray v 11 Boris"
Additionally, you can specify the monsters by a place in the dungeon, in
which case a random monster appropriate for that place will be chosen.
For example:
    crawl -arena "2 Lair:10 v 3 Vault:7"
will choose two random monsters from the 10th level of the Lair and pit them
against three random monsters from the 7th level of the Vaults.
If you want endless hordes of monsters to kill each other forever you can
use the test spawner, an unkillable monster which does nothing but sit there
and summon monsters with the Shadow Creatures spell:
    crawl -arena "test spawner v test spawner"
                           Options and parameters
------------------------------------------------------------------------------
There are three arena parameters that you can set in your crawl options file:
* arena_delay: The delay in milliseconds between turns within the arena.
      Can be set to 0 to make the simulation zip along.
* arena_dump_msgs: If set to true causes messages to be dumped to the file
      arena.result
* arena_dump_msgs_all: If both this and arena_dump_msgs are set to true
      then diagnostic messages will be dumped as well as other messages.
The are also a number a parameters you can use by putting them in the
string which specifies the monsters:
    crawl -arena "parameter1 parameter2 parameter3 goblin v kobold"
The parameters include:
* "arena:name" lets you pick the name of the arena to place the monsters
      in (defaults to "arena:default").  See source/dat/arena.des for
      a list.
* no_force_ai: The monsters in the arena are somewhat smatter than they are
      than when they're facing the player.  This parameter removes that
      extra intelligence.
* names: Causes all non-unique monsters to be given random names.
* no_animate: Prevents corpses from being animated into zombies.
* no_summons: Prevents monsters from summoning more monsters.
* no_chain_summons: Allows the original monsters to summon more monsters,
      but the *summoned* monsters can't themselves summon any more.
* no_immobile: Prevents immobile monsters from being randomly generated.
* allow_zero_xp: Allows zero xp monsters like plants to be randomly generated,
      which are forbidden from the arena by default.
* "arena_place:place" sets the place used when determining which monsters the
      spell Shadow Creatures uses.  So "arena:Lair:5" would make it
      summon the types of monsters you'd find on the 5th lair of the Lair.
      (This defaults to the 20th level of the main dungeon).
* real_summons: Summoned monsters are made real as soon as they're placed in
      the arena, so they can drop corpses and equipment (no_chain_summons
      still prevents them from summoning more monsters, though).
* move_summons: Moves summoned monsters to a new, random location as soon
      as they're placed, so they don't end up clustered around the
      summoner.
* "summon_throttle:N" prevents summoned monsters from being placed if the
      summoner has N or more allies present.
* cycle_random: If any monster summons monsters with the spell Shadow
      Creatures spell (including test spawners) then arena cycles through
      the list of valid monsters, rather than taking rarity into account.
* random_uniques: Any monster using Shadow Creatures will summon uniques.
* "ban_gylphs:ABC" prevents monsters with the listed ASCII glyphs from
      being placed in the arena.  For instance, "ban_glyphs:&C" prevents
      demon lords and giants/cyclopses/titans/etc from being placed.
* "delay:N" allows the delay between turns to be specified on the command
      line instead of in the options file.