[Therion] Showing a lower scrap down a pit in an overlying scrap

[Footleg]

I've been experimenting to try to draw some more complex features in
Therion. One of these is where a passage contains a pit with a second
scrap going off at the bottom of the pit. There is an example of this
in the Therion By Examples pages on the wiki.
http://therion.speleo.sk/wiki/doku.php?id=tbe:wiki3

In that example the lower scrap in included in a separate map, and
that map is then included in the upper map using the preview command:

map map5 -proj plan
   scrap1 at s1
   break
   scrap2 at s2
   preview above map22
 endmap

What is the purpose of using two maps here? I appear to be able to do
this with a single map containing the two scraps.

My second question is whether it is possible to show the pit as a hole
so that the colour of the lower scrap shows in the bottom of the pit
when I colour by altitude. Currently my pit is coloured the colour of
the upper scrap and the colour at the bottom of the pit shows through
the transparent upper scrap. But as there is no floor in the upper
scrap at this point, I want to have no colour shading in the upper
scrap so it is clear that the bottom of the pit is at the level of the
lower passage. See the attached bitmap showing an example of what I
mean (not drawn with Therion). The green lower passage is clearly
shown at the bottom of the pit where the pit floor is coloured green.
Can I do this in Therion?

Footleg
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