diff of two output streams

[root@home tmp]# cp /etc/passwd a; cp /etc/passwd b
[root@home tmp]# sed -i 's/pc_op/pc_ok/' b
[root@home tmp]# diff <(cat a) <(cat b)
33c33
< pc_op:x:777:177:default operator:/home/pc_op:/bin/sh
---
> pc_ok:x:777:177:default operator:/home/pc_op:/bin/sh