The function fts:joinIncludes() is designed to return at most one
element of fst:stringInclude. Although it is OK for it to declare
 'element(fts:stringIncldue)*' as return value, however,
it is more appropriate for it
to be declared to return 'element(fts:stringInclude)?'. 

declare function fts:joinIncludes(
      $strIncls as element(fts:stringInclude)* )
   as element(fts:stringInclude)*
{