Subdivide beams in triplets

[Nick McManus]

Hi all,

I'm having some trouble achieving subdivided beams in this particular 
example:

\version "2.18.2"

\relative c''{
  \set subdivideBeams = ##t
  \set baseMoment = #(ly:make-moment 1 8)
  \tuplet 9/4 {a16 bes a bes c bes c d c}
}

I would like to achieve a set of 9 semiquavers (16th notes) with the beam 
subdivided every three notes.  How do I go about this?

Thanks,

Nick McManus