raspberry_overlay_prompt() {
  if mount | grep -q 'on / type overlay'; then
    REPLY="[OverlayFS] "
  else
    REPLY=""
  fi
}

grml_theme_add_token raspberry_overlay -i raspberry_overlay_prompt '%F{red}' '%f'

() {
  local -a items
  if zstyle -a ':prompt:grml:left:setup' items items; then
    items[$items[(i)path]]=(raspberry_overlay path)
    zstyle ':prompt:grml:left:setup' items "${items[@]}"
  fi
}