[Therion] Join 3 scraps from different maps

[Tarquin Wilton-Jones]

> Instead of joining first the 2 lines of the scrap1.1 and scrap1.2 from
> map1 in th-file1 and then join the 2 lines from scrap1.2 and scrap2.1 in
> the main-th-file I just joined the 3 lines in the main-th file.

A quick quote from the Therion book:
"
When joining more than two points or lines, use one join command for all
of them, not a sequence of join commands for pairs.
E.g. use join a b c, not join a b followed by join b c.
"

Each point/linepoint can only be referenced in a single join command.
Subsequent attempts to use that point/linepoint will be ignored (as you
found out).

> I thought this might be ignored if I only output map1, but the 2 scraps
> are joined correctly.

When you select a subset of your survey, it still processes the entire
survey data, and all scraps for the specific projection, and processes
all the joins correctly. It only *draws* the parts you select, but it
has to process the rest anyway, so that it can pull the lines into the
correct shapes (applying error correction, loop closure warping of
drawings, etc.).

If it didn't work that way, then when you rendered a subset of the map,
the now-disconnected scraps might be rendered differently than when you
render the entire survey, which would be undesirable.

In other words, as you found out; don't worry about where your joins are
specified in the data hierarchy, they will always be used correctly,
even when you are only selecting a subset of the survey below them.