Who's using the UNDO segments

SELECT TO_CHAR (s.SID) || ',' || TO_CHAR (s.serial#) sid_serial,
NVL (s.username, 'None') orauser, s.program, r.NAME undoseg,
t.used_ublk * TO_NUMBER (x.VALUE) / 1024 || 'K' "Undo"
FROM SYS.v_$rollname r,
SYS.v_$session s,
SYS.v_$transaction t,
SYS.v_$parameter x
WHERE s.taddr = t.addr
AND r.usn = t.xidusn(+)
AND x.NAME = 'db_block_size'
/