andrew welch wrote:

><root>
>   <arbitrary />
>   <elem       color="dark-red" />
>   <elem       color="red" />
>   <arbitrary  color="none" />
>   <elem       color="red" />
>   <elem       color="light-red" />
>   <arbitrary />
>   <elem       color="dark-red" />
>   <elem       color="red" />
>   <arbitrary  color="none" />
>   <elem       color="red" />
>   <elem       color="light-red" />
>   <arbitrary />
></root>
[...]
>That is of course if I've scaled up the input correctly, no comment
>from the OP so far.

Yes, this is how the input would scale. I'm sorry for not replying
earlier to the solutions given so far (Andrew, David) - I must admit
that I am still digesting the code given to extract and understand the
underlying idea(s).

For your (Andrew's) solution to example #1, am I right that the
underlying idea could be paraphrased as:

"Group by start, then tail-trim the resulting groups to end (using a
nested grouping from end)"?

I hope I can give a verbal write-up of the ideas behind the solutions
for the other cases as well as soon as I have fully grasped them myself...

-Christian