You are right, id is for line (not only). But line has points numbered: beginning (with yellow tick) is 0, last one is "end".

join line1:1 to line2:end

You may replace this number by "mark". Then you may join mark1 to mark2.

Aug 19, 2013 v 2:20 PM, Graham Mullan <graham.mullan@coly.org.uk>:

However, I note
that the specific ID seems to relate to a line rather than to a single
line point