Join

    for b in bib0/book do 
      for r in review0/book do 
        where b/title/data() = r/title/data() then 
          book [ b/title, b/author, r/review ] 
==> book [
      title  [ "Data on the Web" ], 
      author [ "Abiteboul" ], ...
      review [ "A darn fine book." ] 
    ],
    book [
      title  [ "XML Query" ], 
      author [ "Fernandez" ], 
      author [ "Suciu" ]
      review [ "This is great!" ] 
    ]
:   book [
      title [ String ], 
      author [ String ] {1,*},
      review [ String ]
    ] {0,*}