extra stem

[David Bobroff]

Another puzzle:

I have measure with a string double stop on a single pitch:

\score{


\notes\relative c'{

 as2 a  
}
\paper{}
}

On the second note I want two stems; one going up and one going down.  Is
there a simple solution or do I have to use "voices"?  I've checked the
docs but I'm not sure how to implement the "voice" mechanism within a
single part for one note.

Thanks,

David Bobroff