removeEmptyStaff one by one

[yota moteuchi]

Dear lilypond List

I have something like

\version "2.11.20"
\score {
    \new StaffGroup <<
        \new Staff {
            \voiceA
        }
        \new Staff {
            \voiceB
        }
        \new Staff {
            \voiceC
        }
    >>
\layout { \context { \removeEmptyStaffContext } }
}
\paper { }

It works nice... but I would like to remove the empty staffes (staves ?) for the voices A & B (when empty) but never for the voice C.
Which mean that : even if C is empty (and A or B have some content) : don't remove it

How could I do ?

I already had a look at the archives without finding a nice trick

Yota