How to define letter in custom layout (xkb)?

Question

I'm using Arch Linux.
I'm trying to define, for example, letter 'a' with double grave in xkb custom layout.
It's easy to define, for example, 'a' with grave:

key <AC01> {[ a, A, agrave, Agrave ]};  

This works. I tried something like:

key <AC01> {[ a, A, adoublegrave, Adoublegrave ]};  

It isn't working. Also I want to define a letter with 'inverted breve'.
If someone will, at least, give me an example, it would be greatly appreciated.
Thank you !

Answer 1

You can specify characters by their Unicode code point, so e.g. U0201 and U0203 should be the small a characters with the desired double grave and inverted breve. Use any Unicode table or list to find the others.

For all available key names, see /usr/include/X11/keysymdef.h. There's no name for combined character with double grave or inverted breve though.