How can I selet rows which have 2 columns values cross equal? - Mailing list pgsql-sql

Hi All:
I have a table like this:
  Table test
Id   |  a | b
-----+----+---
1    | 100| 101
2    | 101| 100
3    | 100| 3
4    | 20 | 30
5    | 11 | 13
6    | 3  | 33
7    | 30 | 20

I want to get row 1, 2,4 and 7 selected. Because their values of a and b
are cross equal. i.e., for each pair of rows, 
a.Row1 = b.Row2 and b.Ro1 = a.Row2
a.Row4 = b.Row7 and b.Ro4 = a.Row7

How can I construct a sql statement to select them?
Thanks in advance.
Fay