/etc/bash_completion.d/deborphan is in deborphan 1.7.28.5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 | have deborphan &&
_deborphan()
{
  local last_item_in_list
  local cur=$2 prev=$3
  local options='--help -h --version -v --status-file -f
    --show-deps -d --show-priority -P --show-size -z
    --show-section -s --no-show-section --nice-mode -n
    --all-packages -a --libdevel --exclude -e --priority -p
    --force-hold -H --find-config --add-keep -A --del-keep -R
    --list-keep -L --zero-keep -Z --keep-file -k
    --guess-perl --guess-python --guess-pike
    --guess-ruby --guess-interpreters
    --guess-section --guess-dev --guess-debug
    --guess-common --guess-data --guess-doc --guess-dummy
    --guess-all --guess-only
    '
  COMPREPLY=()
  case "$prev" in
    # previous option asks for a file
    -@(-status-file|f|-keep-file|k))
      _filedir
      return 0
      ;;
    # previous option asks for a package or "-"
    # if there's more than 1 package listed, use the default below
    # for package completion
    # could maybe use /var/lib/deborphan/keep for -R|--del-keep
    # but tricky if -f|--status-file is given
    -@(-add-keep|A|-del-keep|R))
      COMPREPLY=( $( compgen -W "$( _comp_dpkg_installed_packages $cur ) -" -- $cur ) )
      return 0
      ;;
    # previous option asks for a priority
    -@(-priority|p))
      COMPREPLY=( $( compgen -W '1 2 3 4 5 required important standard optional extra' -- $cur) )
      return 0
      ;;
    # previous option asks for a commaseparated list of packages
    -@(-exclude|e))
      last_item_in_list="${cur##*,}"
      list_before_item="${cur%$last_item_in_list}"
      #add "," to the list and take care of already existing listed packages
      COMPREPLY=(
                  $(
                   compgen -S "," -P "$list_before_item" -W "$( _comp_dpkg_installed_packages $last_item_in_list )"
                  )
                )
      return 0
      ;;
  esac
  if [[ "$cur" == -* ]]; then
    # return one of the possible options
    COMPREPLY=( $( compgen -W "$options" -- $cur ) )
  else
    # return matching installed packages
    # (default and for -A or -R package lis
    COMPREPLY=( $( _comp_dpkg_installed_packages $cur ) )
  fi
  return 0
}
[ -n "${have:-}" ] && complete -F _deborphan $filenames deborphan
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