[Therion] Revisiting Breaking extended elevations on specific stations

[Marco Corvi]

last weekend i completed the survey of a cave with some parallel close pits
connected through several "windows".
the last survey joined the previous surveys at seven stations.

needless to say, i was not able to tell therion how to break the centerline
for the extended elevation. Eventually i removed a few "equates"
(the loops closed quite well anyways)

extended elevation requires a spanning tree of the centerline graph.
"extend" is meant to tell therion how to break loops "removing" edges/legs.

I suspect that this semantics of "extend stop" is not sufficient because if the
user moves the data around (which is acceptable because in therion data can
be reordered) the result could be different.

I believe that we need a way the user tell therion how to construct the spanning
tree without ambiguities. The only solution i can see is telling which
edge to remove.
The drawback is that each station appears only once (at one node) in
the tree, and
there cannot be two stations "7" as in Tarquin's problem.

back to the problem i experienced:
what about an option to "equate" to tell therion that it must not be
used for the
extended elevation ?

marco