Mixed Staff Sizes

[Gilles]

Hello.


The piece consists in a "Flute" part and a "Piano" part.

In one file (for the flute), I have:
-----
flautoNotes = \notes \relative c'' {
    \time 3/4
    e8 d16 c b a g fis e8 c' |
}

flautoStaff = \context Staff = "Flauto" {
    \set Staff.instrument = \markup \column < "Flauto" "traverso" >
    \set Staff.midiInstrument = "recorder"
    \notes { \flautoNotes }
}
-----

Similarly in a second file for the piano part.

And, in a third file, the combined parts:
-----
\include "flauto.inc.ly"
\include "piano.inc.ly"

\score {
    \context StaffGroup <<
        \context Staff = "Flauto" { \set Staff.fontSize = #-4 }
        \flautoStaff
        \pianoStaff
    >>
}
-----

In the combined part, I'd like to have the "Flute" staff typeset
in a smaller font; the "\set" command, above, indeed reduces the
font of the objects in the the staff, but not the staff itself!
[Hence the music stills takes the same total amount of vertical
space.]

I tried to add in the other settings, as shown in the example 
"staff-mixed-size.ly" (in "Regression Tests"). But this still
doesn't achieve the desired result (all staves become smaller).

I think that I can't use the syntax exactly as in that example
("\with") because I want the separate "Flute" part to be typeset
at the normal size, so the size modifying commands should be in 
the combined staves file.

What is the right way to go?


Thanks and best regards,
Gilles