\layout {
  \context {
    \Voice
    \consists "Horizontal_bracket_engraver"
    \override HorizontalBracket.direction = #UP
  }
}

{
  \once\override HorizontalBracketText.text = "a"
  c''\startGroup d''\stopGroup
  \once\override HorizontalBracketText.text = "a'"
  e''\startGroup d''\stopGroup
  c''
  -\tweak HorizontalBracketText.text \markup \bold \huge "b" \startGroup
  -\tweak HorizontalBracketText.text "a" \startGroup
  d''\stopGroup
  e''-\tweak HorizontalBracketText.text "a'" \startGroup
  d''\stopGroup\stopGroup
  c''-\tweak HorizontalBracketText.text foo \startGroup d'' e'' f''
  \break
  g'' a'' b'' c'''\stopGroup
}