Help for \beamExceptions

[Richard Shann]

I've been trying to use the music function \beamExceptions which is in
version 2.19.25, I can't find anything much to document it though.
For example, trying this

\version "2.19.25"

MvmntIVoiceI = {
         e'8[ f'8 g'] a'8\noBeam b'8 a'[ g'8 f'] |
         e'8 f' g' a' b' a' g' f' 
}

\score { 
        \new Staff {\time 4/4 \MvmntIVoiceI } 
        \layout {
                \set beamExceptions = \beamExceptions { \time 4/4  e' 8[ f' 8
g' ] a' 8\noBeam b' 8 a' [ g' 8 f' ] }
        }
      }

the second bar is not beamed like the first, despite the music argument
to \beamExceptions being a copy of the first bar.

Is the next stop some source file for finding out what can be done, or
am I missing some documentation?

Another topic that came up while I was doing this:
I have used \set beamExceptions here, but I couldn't figure out what is
allowed for setting the context - Voice and Staff worked but \set
Score.beamExceptions was ignored, and I couldn't find out what \set
Timing.beamExceptions was doing - Timing doesn't appear in the hierarchy
of contexts...

Richard