book and instrument

[Jean-Charles Malahieude]

Hi all!

When using a separate book for generating instrumental parts, is there a
easy way to get rid of the superfluous "instrument" appearing with each 
bookpart?

---8<---
\version "2.13.61"
\book {
\header {
  title = "The Book Title"
  instrument = "instrument"
}
\bookpart {
\header {
  title = "Act one"
}
\score {
  \new Voice { c'4 }
  \header {
    piece = "piece 1"
  }
}
\pageBreak
\score {
  \new Voice { c'4 }
  \header {
    piece = "piece 2"
  }
}
}%ends bookpart
\bookpart {
\header {
  title = "Act two"
}
\score {
  \new Voice { c'4 }
  \header {
    piece = "piece 3"
  }
}
}%ends bookpart
}%ends book
--->8---

TIA,
Jean-Charles