Hi everyone,

Please find attached a patch adding hierarchy.el and
hierarchy-tests.el. The hierarchy library allows to create, query,
navigate and display hierarchy structures.

Two years ago, I suggested adding this library to ELPA and you guys on
CC answered you wanted it in Emacs core:
https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00624.html

You also had several suggestions and I implemented none of them. I agree
they all make sense, I just haven't found motivation to do that yet
because hierarchy is working well for me already.

What do you think?

-- 
Damien Cassou

"Success is the ability to go from one failure to another without
losing enthusiasm." --Winston Churchill