You maybe don't want to start by writing to a file.  You want to separate "did the query produce something sensical" and "does the file write do what I expect" as sources of error so you can't try to debug the wrong thing for a few hours.

return
(: file:write($output || $filename, :)

local:process-data($data)

(: ) :)

can be a remarkably useful construct until you like what you're seeing.  The BaseX GUI very sensibly truncates the output so you can't crush it.

-- Graydon