Version 2.0.3 of the DocBook XSLT2 stylesheets don't know the @function attribute.
For example, the following code creates an empty string:
I would have expected to see an Alt key in the corresponding language. Not every key on the keyboard is the same. For example, the control key is abbreviated Ctrl in English, but Strg in German.
Using an empty keycap and automatically insert the relevant string makes it consistent.