explicit 7 on a half diminished 7

[Derek Klinge]

For the life of me I cannot figure this out.

I would like the following example to explicitly include the 7. Is there a way to change the way it handles half diminished 7ths?

EXAMPLE

\chords {
  c4:7.9- c:7.9-/g
  \set slashChordSeparator = \markup { " over " }
  \break
  c4:7.9- c:7.9-/g
}

Derek