String number collision

[Bobber]

I'm engraving some guitar music.  It's polyphonic on one staff.  In one 
measure I am indicating string numbers.  The numbers are colliding with 
the upper voice.  Here's the measure where it's happening:
> % measure  20:
>       <<
>          { r8 <c'\3>8 <e'\2>8 c'8 e'4 }  \\
>          { c2. }
>       >>
The eight notes here have the stems pointing upwards.  The numbers 
appear to be placed right above the note heads and are colliding with 
the stems.

I'm kind of new to Lilypond so I was wondering if someone could give me 
some pointers and help me resolve this.

Thanks!

--
Bob Wooldridge
Blog: http://kc0dxf.net/blog/