# complete -C foo foo

# add functions beginning with x. to COMMANDS
while IFS= read -r line; do
  [[ $line =~ ^declare\ -f\ x\. ]] || continue
  COMMANDS+=( "${line##declare -f x.}" )
done < <(declare -F)
mapfile -t COMMANDS < \
  <(LC_COLLATE=C sort < <(printf "%s\n" "${COMMANDS[@]}"))

# complete if completion context detect (complete -C kapp kapp)
if [[ -n $COMP_LINE ]]; then
  line=${COMP_LINE#* }
  for c in "${COMMANDS[@]}"; do
    [[ ${c:0:${#line}} == "${line,,}" ]] && echo "$c"
  done
  exit
fi

# delegate to COMMAND passed as first argument
if [[ -n "$1" ]]; then
  declare CMD="$1"; shift
  for c in "${COMMANDS[@]}"; do
    if [[ $c == "$CMD" ]]; then
      "x.$CMD" "$@"
      exit $?
    fi
  done
fi