Selection

    for b in bib0/book do 
      where b/@year/data() <= 2000 then
          b 
==> book [
      @year  [ 1999 ],
      title  [ "Data on the Web" ],
      author [ "Abiteboul" ],
      author [ "Buneman" ],
      author [ "Suciu" ]
    ] 
:   Book{0,*}