Problem understanding subdivided beam

[Urs Liska]

Hi,

I have a problem getting a beam subdivision right, and I can't seem to
find a solution in the manual:

I have this group of 16th notes:

\relative b {
  \time 6/4
  b16 [ d b' g  d g d' b  g b g' d ]  r2.
}

and I want to have the secondary beam divided in three parts with one
quarter note each.

However
\set subdivideBeams = ##t

breaks the beam in three groups altogether, and no combination of
baseMoment and beatStructure I could think of produced the expected result.

Any suggestion?

TIA
Urs