4-voice, 3-staff score with 2 lines of lyrics

[CSÉCSY László]

Hi Lilypond people,

I have been trying this stuff to render well since days, digging deeper
and deeper into the docs, but without success. Attached is an extract to
show my progress and to get help.

The beginning is a 7-measure intro (played on piano), the last measure
shown to the choir. The problem is: I can not get the "ABCDEFGH" lyrics
line rendered, which is going to be the lyrics for both soprano and alt.
(Tenor and bass has the same, "abcdefgh" lyrics, which is rendered
right.) I enclosed those into a \book because there are going to be some
unisono parts as well, which are a snap.

I think the whole problem is that I have single staff poliphony (soprano
and alt on the same staff, but not as chords) versus only one line of
lyrics.

So please help me: what should I write into that .ly to have it rendered
as it should?
--
Bye
Boobaa
---------------------
http://boobaa.ajrg.hu

\version "2.7.14"

#(set-global-staff-size 20)

PartASopAltStaff = \relative c' {
        \set Staff.instrument = \markup { \column { S A } }
        \clef violin
        \time 4/4
        \override MultiMeasureRest #'expand-limit = 1

        R1*6 |
        << {
        c8 d e f g a b c |
        e,4 f g a | b c d e
        } \\
        {
        s1 |
        c,4 d e f | g a b c
        } >>
}

PartATenStaff = \relative c' {
        \set Staff.instrument = #"T"
        \clef violin
        \time 4/4
        \override MultiMeasureRest #'expand-limit = 1

        R1*6 | R1 |
        g4 a b c | d e f g
}

PartABasStaff = \relative c {
        \set Staff.instrument = #"B"
        \clef bass
        \time 4/4
        \override MultiMeasureRest #'expand-limit = 1

        R1*6 | R1 |
        c4 d e f | g a b c
}

PartASopAltText = \lyricmode {
                _ _ _ _ _ _ _ _
                A B C D E F G H
}

PartATenText = \lyricmode {
                a b c d e f g h
}

\book {
        \header {
                title = "4-voice 3-staff"
        }
        \score {
                \context StaffGroup <<
                        \context Staff = cStaffAA <<
                                \context Voice = cStaffAA \PartASopAltStaff
                        >>
                        \context Lyrics = cStaffAA \lyricsto cStaffAA 
\PartASopAltText

                        \context Staff = cStaffAB <<
                                \context Voice = cStaffAB \PartATenStaff
                        >>
                        \context Lyrics = cStaffAB \lyricsto cStaffAB 
\PartATenText

                        \context Staff = cStaffAC <<
                                \context Voice = cStaffAC \PartABasStaff
                        >>

                        \set Score.skipBars = ##t
                >>
                \midi { \tempo 4 = 100 }
                \layout {
                        indent = 0\cm
                }
        }
}