D7 chord symbol

[Helgi Örn Helgason]

Hello all!
I can't find in the documentation any hint on how to write a major  
chord symbol like for example D7. Can someone tell me how this is  
done or point that out for me in the documentation?
The d below is supposed to be a D7 in the output:
}

harmonies = \chordmode {
        r4 g5 d
}

Thank's,
Helgi Örn