\transpose should not affect \transposition

[Trevor Daniels]

The \transpose command transposes all pitches within its scope, 
including the pitch supplied to the \transposition command.  This 
causes incorrect MIDI output for transposing instruments.  The pitch 
supplied to \transposition is a constant determined by the 
instrument and should never be transposed.

\score {
 \transpose c d {
  \new StaffGroup <<
    \new Staff {
      \key c \major
      c'1
    }
    \new Staff {
      \transposition bes
      \key d \major
      d'1
    }
  >>
 }
 \layout {}
 \midi {}  % Should be a unison D, but is not
}

Trevor