Extra staff in score with lyrics

[liang seng]

Hi, I'm using Lilypond 2.7.17 to do a score with solo voice and strings. 
Here
is a small extract from the voice line (lyric.ly) :

\version "2.7.17"
lyric =
\relative
<<
 \context Staff \context Voice = "melody" {
    f'2 << \context Voice = other { \voiceOne \tiny g c f,4 } \context
Voice=melody { \voiceTwo \normalsize g,2 c f,4 } >> \oneVoice r4
 }

 \lyricsto "melody" \new Lyrics { Al -- le -- lu -- ja. }
> >

And here is the full score (score.ly):

\version "2.7.17"
\include "lyric.ly"
\score {
<<
\new StaffGroup <<
\new Staff << { \set Staff.instrument = "Violin" R1 R1 } >>
\new Staff << { \set Staff.instrument = "Viola" R1 R1} >>
\new Staff << { \set Staff.instrument = "Soprano" \lyric } >>
\new Staff << { \set Staff.instrument = "Cello" R1 R1 } >> >> >>
}

For the sake of simplicity, all the staves except the soprano staff contain 
only rests.
Here, when I compile score.ly, the output look good except for one thing. 
The
soprano staff is at the correct position (3rd staff from the top). However, 
the
lyrics, which are supposed to be below the soprano staff, are at the bottom 
of
the score, below the last (cello) staff. Why is this? How can I rectify 
this?
Another interesting thing is, when I replace \context Staff with \new Staff 
in
lyric.ly, the full score has an extra staff above the soprano staff, but the
staff has no notes or rests, only a clef. Again, why does this happen?
Thank you.
Seng Liang.