How to obtain algebraic sum of equal lines

Hi all,
Is it possible, given the following table:

cod_rif | titolo | quantity | cmf | u_qq | mont | vend | sum
---------+--------+----------+---------+--------+---------+------+-------
26 | aaa | -1000 | 6.11098 | 6.176 | 6.1110 | t | -1000
7 | aaa | 2500 | 25.455 | 0 | 60.0897 | f | 2500
28 | bbb | 2700 | 3.862 | 4.6 | 4.1957 | f | 2700
3 | bbb | 500 | 6.057 | 10.129 | 18.7311 | f | 500
34 | bbb | -700 | 0 | 0 | 0.0000 | t | -700
6 | bbb | -500 | 2.703 | 4.757 | 3.7151 | f | -500
30 | ccc | 5000 | 1.717 | 1.489 | 1.7170 | f | 5000
33 | ccc | 10000 | 4.36 | 0 | 4.3600 | f | 10000
11 | ddd | -1500 | 10.537 | 4.021 | 20.5815 | t | -1500
32 | ddd | 1500 | 0.55896 | 1.119 | 0.5590 | f | 1500
(10 rows)

to get a selection whereby the algebraic sum of "quantity" for each equal 'titolo'
is returned?

I tried this query with no result:
psql finanza -c "select * , (sum(quantity)) from test_t group by cod_rif, titolo, quantity, cmf, u_qq, mont, vend order by titolo asc"

Regards,
Ennio.

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