\transposition does nothing?

[Jérôme Plût]

I am typing a horn part in F. In the attached file, \transposition
does nothing. Where should I put it: at the voice, Staff, Score level?
The documentation 
(http://lilypond.org/doc/v2.19/Documentation/notation/displaying-pitches#instrument-transpositions)
 is very unclear about this.

The file itself:


\version "2.19"

Ig = \relative {
  \time 4/4 \key g \major \partial 4
}
IcA = \transpose c g \relative {
  r4 R1 r8. c'16\f c4 r2 R1 d4\f r r2 R1*2 e2\p(^"soli" d) c4 r r2 R1*2 R1*5
}
\bookpart { \header { instrument = "Cor I (Fa)" }
\score { { \new Staff { \transposition f << \Ig \transposition f \IcA >> } } }
}
% note that this outputs a score in g major and not, as it should,
% in d major



Thanks,

-- 
        Jérôme