Defining a random monster set
-----------------------------
Portal vaults require a defined random monster set to make the Shadow
Creatures spell work. This is done by calling dgn.set_random_mon_list()
manually. Here's an example from ice_cave_small_02 in icecave.des:
  : dgn.set_random_mon_list("ice beast w:90 / ice dragon / nothing")
You can use "nothing" to have the spell fail sometimes.
If you are using the same random monster list in several destination maps,
you can define a lua block and call it from the destination map definition.
This example is from sewer.des:

{{
function sewer_random_monster_list(e)
  e.set_random_mon_list("giant bat w:20 / giant newt w:20 / small snake / \
                         ooze / worm / snake / giant mosquito w:15")
end
}}

You can then use this line in the map definition to execute the lua block:
  : sewer_random_monster_list(_G)
You can also set env.spawn_random_rate() to have monsters generated from the 
list during play.