There’s a link to one of the related files from your link, but folks might also find the rest of my article about this from 2006 interesting:

http://www.hcs.harvard.edu/~jrus/site/cocoa-text.html

Also cf.:

http://www.hcs.harvard.edu/~jrus/site/system-bindings.html

http://www.hcs.harvard.edu/~jrus/site/selectors.html

https://github.com/jrus/cocoa-text-system/tree/master/KeyBin...