At the end of the last function invocation of the supplied expression for https://www.w3.org/TR/xpath-functions-31/#func-map-merge, there seems to be a closing parenthesis ')' missing. 

Relevant part:

  [...]
  return fn:fold-left($MAPS, map{}, 
      $combine-maps(?, ?, $duplicates-handler(($OPTIONS?duplicates, "use-first")[1]))
                                                                                     ^here