\transpose and \transposition combination error(?)

[Zbyněk Burget]

Hi,
I found a (probably) error in combination \transpose and \transposition 
functions.
I've score:

\score {
  \new StaffGroup <<
    \new Staff {
      \key c \major
      c'1
    }
    \new Staff {
      \transposition bes
      \key d \major
      d'1
    }
  >>
  \midi {}
}

There is allright. But...

...now I need transpose all score to another key:

\score { \transpose c d {
  \new StaffGroup <<
    \new Staff {
      \key c \major
      c'1
    }
    \new Staff {
      \transposition bes
      \key d \major
      d'1
    }
  >> }
  \midi {}
}

In this case is produced midi file wrong. Only way to produce correct 
midi is as follows:

...
    \new Staff {
      \transposition bes
      \key d \major
      \transpose c g { d'1 }
    }
...

Is it error or feature?

Zbynek