score layout

[Seng Liang]

Hi, I am typing a score for a work for voice and strings and I have a problem 
with the score layout. I am using Lilypond version 2.7.12 for Windows (98). 
Here is the file: 

\version "2.7.12" 
\relative {

\new ChoirStaff << 
\new GrandStaff <<
\new Staff << { \set Staff.instrument = "Violins 1" c1 } >>
\new Staff << { \set Staff.instrument = "Violins 2" c } >> >>
\new Staff << { \set Staff.instrument = "Viola" \clef alto c } >>
\new Staff << { \set Staff.instrument = "Voice" c } >>
\new Staff << { \set Staff.instrument = "Bass" \clef bass c, } >> >> }


As you can see, the score layout used is the old-styled layout, where the voice 
staff lies between the viola and the bass. This layout is perfect except for 
one thing; How do I connect the vertical barlines between the viola staff and 
the violin staff above? The barline break between the viola and voice staff, as 
well as between the voice and bass staff, is intended though.
Thank you very much.
Yours sincerely,
Seng Liang.