Dear Jan,
The import worked well, but when calling functxyz:capitalize-first("test") I got result: Error: [FONS0004] No namespace declared for prefix 'functx'.
that's probably related to sth. listed in the following GitHub issue..
https://github.com/BaseXdb/basex/issues/133
..which is now the oldest entry in our list. a strong indicator this should be addressed soon..
Thanks or asking, Christian