trembling tremolos

[Aaron]

HI all,
I am try to enter a measure with tremolo beams but I can't figure out 
how, as follows
a quarter note with a 16th tremelo on the beam:

\repeat "tremolo" 4 c16 (from the documents)
gives me this:
----------snip----------
/home/aamehl/Documents/viola.ly:23:21: warning: Chord tremolo with 1 elements. 
Must have two elements.:
       \repeat "tremolo" 4
                           d''8 f'' e''4\) d''2. r4 |
programming error: Skipped something?
Grob StemTremolo ended before I expected it to end. (Continuing; cross thumbs)
programming error: Skipped something?
Grob StemTremolo ended before I expected it to end. (Continuing; cross thumbs)
programming error: Skipped something?
------------------------------snip---------------
so I tried this:
\repeat "tremolo" 4 {c16 }
which gave  me:
------------------snip----------------
Interpreting music...
/home/aamehl/Documents/viola.ly:23:48: warning: barcheck failed at: 7/8:
       \repeat "tremolo" 4 {d''16} f'' e''4\) d''2. r4
                                                      |
-------------------------snip--------------

so how do I write a stem tremolo on one note?
Thanks
Aaron