case "$THIS_SUB_DEPENDS" in
  VOLUME_ID) if ! list_find "$_EXTRAS_TARG" "extras/volume_id"
             then
               echo "Sub-dependency VOLUME_ID requested, adding build"  \
                    "target 'extras/volume_id'."                        &&
               _EXTRAS_TARG="$_EXTRAS_TARG extras/volume_id"            &&
               persistent_add  _EXTRAS_TARG
             fi
             ;;
          *) echo "Unknown sub-dependency $THIS_SUB_DEPENDS!"
             return 1
             ;;
esac
