volta repeat challenge

[Stan Mulder]

I want to repeat a 16 bar comping section. 

I can get it to write the slash marks, but the repeat symbols won't show up. I'm
using the "comp" macro found at lilypond.org (I think) which writes the slash
marks. I can post that if necessary.

\repeat volta 2 {
        <<
                \comp #64
                s1*16^\markup {SOLOS}
        >>
}

If I insert the repeat bars manually with \bar "|:" and \bar ":|" only the
opening  repeat sign appears.

The chords are defined separately and get inserted at another place.