I implemented a small change to Abandoned mines that has to do
with the requirements you need in order to abandon the mine, visible
with 'tunnel evaluate mine'.

In order to break the monotony of the structures that are currently
popular, there are a few changes:

- You need to build more dead ends on each direction.
* Keep checking this actively, because you any time you
dig, that's a dead end that will count, but as soon as you
go forwards it is no longer one and is not counted anymore.

- The minimum depth is lowered to 5 (from 10)

- The difficulty of the mine is not determined by the
depth, but the number of rooms (never has been but some
seem to have the idea).
That is you can just as well stay closer to the ground and not run
into many obstacles while doing the mine.

- The decorations of the rooms reflect the change, so that more variety
is in the 0-20 depth range.

Please note that this can be rolled back quickly if it doesn't work.
Let's see.

~/A