[Therion] cave depth

[Bruce]

> Well the depth could become +15, -230, rather than just 245m if there
is one entrance 15m below the highest point. You may or may not prefer
that - just saying that they could be related in that way.

Yes I agree, there are other ways of thinking about definition of cave
depth, but not yet able or likely to be automated using Therion.  Although
the recent \cave[max|min]z variables are a step in allowing users to
customise in that direction.

Bruce