$ ID=$(journalctl --new-id128 | sed -n 2p)
$ printf $'<5>\n<6>\n<7>\n' | systemd-cat -t "$ID" --level-prefix true
$ journalctl --flush
$ journalctl -b  -t "$ID" -q
Nov 17 21:50:53 wily-64 d5d77f0a779d4c80a40b45434ad44571[21535]:
Nov 17 21:50:53 wily-64 d5d77f0a779d4c80a40b45434ad44571[21535]:
Nov 17 21:50:53 wily-64 d5d77f0a779d4c80a40b45434ad44571[21535]:

# should be nothing

$ ID=$(journalctl --new-id128 | sed -n 2p)
$ printf $'\tfirst\n\t\tsecond\n' | systemd-cat -t "$ID" --level-prefix false
$ journalctl --flush
$ journalctl -b -o cat -t "$ID"
first
second

# should be
#    first
#        second