putting piano centered dynamics into score

[liang seng]

Hi, I am using Lilypond 2.7.27 for Windows 98 and I have a problem putting a 
file which contains piano centered dynamics into a score. The contents of 
the file "piano.ly" are the same as the input file used to generate piano 
centered dynamics in the 2.7 documentation file. The file of the score is as 
below:
\version "2.7.27"
\include "piano.ly"
\score{
\new StaffGroup << \override Score.InstrumentName #'space-alist =
#'((left-edge extra-space . 4.0))

\new Staff { \set Staff.instrument = "Violin" R2 }
\new Staff { \set Staff.instrument = "Viola" R2}
\new PianoStaff << \set PianoStaff.instrument = "Piano"
\new Staff { \upper }
\new Staff { \lower } >>

\new Staff { \set Staff.instrument = "Bass" R2 }
> >
}

The contents of \upper are:
\new Voice \relative c {
      \clef treble
      \key c \major
      \time 2/4
    \change Staff=lower
r8 g32 b d g a8\rest \stemUp g32 b \change Staff=upper d g   }

The contents of \lower are:
\new Voice \relative c  { \clef bass
\stemDown \times 2/3 { g,16 b d } c8\rest \times 2/3 { g'16 b d } r8
    }

The problem is highlighted in the pdf file. The piano staff appears twice 
while the piano staff which is in the correct position does not display the 
manual staff switches. What is wrong?

Testb.pdf
Description: Adobe PDF document