beaming and staff changes

[Dave]

I am trying to write a line of eighth notes which start in the bass, move to the treble, and back again -- keeping them beamed together.  Here's an example of what I thought would work:

 

\translator Staff = treble ees' | [f g a \translator Staff = bass a,] bes4 c |

 

lilypond bombs out however with a bunch of these errors:

 

programming error: Beams are not left-to-right (Continuing; cross thumbs)

 

I can do this fine:

 

\translator Staff = treble ees' | [f g a] \translator Staff = bass a, bes4 c |

 

but that doesn't include the a, in the beaming.

 

Any ideas on what I'm doing wrong?