Analysis brackets

[Hamish Allan]

Hi,

As this is my first post, I'd like to say thank you very much to the 
developers for creating Lilypond -- I've just yesterday started having 
to typeset music within LaTeX documents; I had a quick play with 
MusiXTeX, but thankfully shortly afterwards found Lilypond and 
immediately became very glad of its existence!

My query is about analysis brackets. I'm trying to show nested metrical 
structure in Bach's Prelude No. 1 from the Well-Tempered Clavier (see 
source code below). I want two brackets per bar, then brackets round 
pairs of those, etc. I'm using the PianoStaff so I've got two staves. 
If I put the groups around notes in the top stave. the brackets appear 
between the staves, wheras I want them either below or above both 
staves. If I put the groups around the notes in the bottom stave, the 
brackets don't stretch far enough (because the notes in the bottom 
stave are longer - but I want the brackets effectively to cover both 
staves). I tried adding eight s16 invisible rests to the chord at the 
bottom but the brackets would still only stretch as far as the visible 
notes.

Is there any way either to have the brackets upside down above the top 
stave, or to have them cover more of the bottom stave?

Whilst we're here, can I use the square bracket shorthand for 
\startGroup and \stopGroup without changing the time signature? I tried 
it out, but of course it just extended the beams over the rest. Also, 
how might I use the triangular grouping engraver?

Many thanks in advance,
Hamish

--------

\paper {
  linewidth = 413\pt
  indent = 0.0\mm
  raggedright = ##t
  #(paper-set-staff-size (* 18.0 pt))
  \context {
    \StaffContext \consists "Horizontal_bracket_engraver"
  }
}

\score {
  \notes \context PianoStaff <<
    \context Staff = "up" {
      \relative c'' {
        r8\startGroup\startGroup\startGroup\startGroup g16 c e g, c 
e\stopGroup
        r8\startGroup g,16 c e g, c e\stopGroup\stopGroup
        r8\startGroup\startGroup a,16 d f a, d f\stopGroup
        r8\startGroup a,16 d f a, d f\stopGroup\stopGroup\stopGroup
        r8\startGroup\startGroup\startGroup g,16 d' f g, d' f\stopGroup
        r8\startGroup g,16 d' f g, d' f\stopGroup\stopGroup
        r8\startGroup\startGroup g,16 c e g, c e\stopGroup
        r8\startGroup g,16 c e g, c 
e\stopGroup\stopGroup\stopGroup\stopGroup
      }
    }
    \context Staff = "down" {
      \clef bass
      \relative c' {
        << c2 \\ { r16 e8. ~ e4 } >>
        << c2 \\ { r16 e8.~ e4 } >>
        << c2 \\ { r16 d8. ~ d4 } >>
        << c2 \\ { r16 d8.~ d4 } >>
        << b2 \\ { r16 d8. ~ d4 } >>
        << b2 \\ { r16 d8.~ d4 } >>
        << c2 \\ { r16 e8. ~ e4 } >>
        << c2 \\ { r16 e8.~ e4 } >>
      }
    }
  >>
}