This aspect of XQuery syntax (when to use commas, when to construct explicit sequences with wrapping parens) can be a bit challenging to internalize.
Thanks for your constructive feedback.
I hope that further clarifications will adjust known construction challenges.
This rework runs for me in the BaseX GUI:
I observed also software behaviour for this adjusted script variant which is acceptable finally.
* How much were the error messages (which I mentioned for the discussed use case) misleading?
* Should the desired document addition work also by the application of a corresponding inline function expression? https://docs.basex.org/wiki/XQuery_Update#User-Defined_Functions
Regards, Markus