Iteration

    for b in bib0/book do
      book [ b/author, b/title ] 
==> book [
      author [ "Abiteboul" ],
      author [ "Buneman" ],
      author [ "Suciu" ],
      title  [ "Data on the Web" ]
    ],
    book [
      author [ "Fernandez" ],
      author [ "Suciu" ],
      title  [ "XML Query" ]
    ] 

:   book [
      author[ String ]{1,*},
      title[ String ]
    ]{0,*}