Where are X11 KeySym defined?

I'm following this tutorial explaing that I can refer characters with their unicode, or with their name defined inside /usr/include/X11/keysymdef.h. However, this file doesn't exist on my (Debian) system. I ran a search with sudo grep -rl "Greek_alpha" inside /usr, but it gave me irrelevant results, nothing like the definitions I'm looking for. (Greek_alpha is one of the aliases I'm looking for, you cand find one in usr/share/X11/xkb/symbols/gr for example.) Where can I find the complete list of these KeySyms / variables / aliases on my computer?