Custom selections (in respect to ICCCM 2.6.1)

[schrmh]

Currently, xsel only allows to copy to and paste from the three standard selections.
However, ICCCM 2.6.1 mentions that more selections can exist:

There can be an arbitrary number of selections, each named by an atom. To conform with the inter-client conventions, however, clients need deal with only these three selections:

PRIMARY
SECONDARY
CLIPBOARD

Other selections may be used freely for private communication among related groups of clients.

(https://tronche.com/gui/x/icccm/sec-2.html#s-2.6.1)

I wish for xsel to support selections of custom names.

This blog post might help a bit:
https://www.uninformativ.de/blog/postings/2017-04-02/0/POSTING-en.html