LilyPond: how to correctly stack rests and tuplet brackets for different voices?

Question

Here is the original fragment I am trying to engrave:

Here is my naive approach:

\version "2.24.3"
\language "italiano"

\new Staff <<
    \relative do' {
        \key reb \major
        \numericTimeSignature
        \time 9/8
        \clef bass
        <<
            {
                s4. <do mib>4. <mib solb>4.
            } \\
            {
                \tuplet 6/9 {r8 solb, [lab do sib solb]} |
            } \\
            {
                \voiceTwo
                <lab, lab,>2.
            }
        >>
    }
>>

The result is puzzling:

How can I fix it?

Update

After a lot of documentation search, and experimenting, here is my best version so far

\version "2.24.3"
\language "italiano"

\new Staff <<
    \relative do' {
        \key reb \major
        \numericTimeSignature
        \time 9/8
        \clef bass
        <<
            {
                \tupletDown
                \override Stem.direction = #DOWN
                \tuplet 6/9 {
                    r8 solb _[lab do sib solb]
                } |
            } \\
            {
                \override Stem.direction = #UP
                s4. <do mib>4. <mib solb>4.
            } \\
            {
                \override Stem.direction = #DOWN
                <lab,,, lab'>2.
            }
        >>
    }
>>

Any idea how to shift the first chord so it doesn't interfere with the tuplet?

Answer 1

The upper voice in a staff should be \voiceOne and the lowest voice should be \voiceTwo. Middle voices should be three and four, where \voiceThree has stems up and \voiceFour has stems down.

However, if we try setting the middle voice to be \voiceFour, its rest and tuplet bracket want to go below the lowest voice to avoid a collision. We can fix this by instead setting the middle voice to be \voiceThree, and the stems and tuplet down (with \stemDown and \tupletDown respectively). This works, but the tuplet bracket ends up overlapping with the stem of the lowest voice (just like in your update):

To remedy this, we can horizontally offset the rest with \once \override NoteColumn.X-offset = #2.5:

…
        <<
            {
                \voiceOne
                s4. <do mib>4. <mib solb>4.
            } \\
            {
                \voiceThree 
                \stemDown \tupletDown
                \once \override NoteColumn.X-offset = #2.5
                \tuplet 6/9 {r8 solb,[ lab do sib solb]} |
            } \\
            {
                \voiceTwo
                <lab, lab,>2.
            }
        >>
…

---

If you don’t like the large horizontal offset, another option would be to change the position of the tuplet beam with \once \override Beam.positions = #'(0 . 0), and to beam under the rest \tuplet 6/9 {r8[ solb, lab do sib solb]}. This will make the tuplet bracket necessary: