Cumulative running total

I have the following table:

Name              Null?    Type
----------------- -------- ------------
A                          VARCHAR2(6)
B                          VARCHAR2(6)
YEAR              NOT NULL NUMBER(38)
PERIOD            NOT NULL NUMBER(38)
AMOUNT            NOT NULL NUMBER(26,3)

and the following content:

SQL> select * from test where year = 2008 and a = 'BV';

A      B            YEAR     PERIOD     AMOUNT
------ ------ ---------- ---------- ----------
BV     BS           2008          1        100
BV     AS           2008          2        200
BV     BS           2008          3        300
BV     AS           2008          4        400
BV     BS           2008          5        500
BV     AS           2008          6        600
BV     BS           2008          7        700
BV     AS           2008          8        800
BV     BS           2008          9        900
BV     AS           2008         10       1000
BV     BS           2008         11       1100
BV     AS           2008         12       1200
BV     BS           2008         13       1300
BV     AS           2008         14       1400
Note that there are gaps for each B. Here BSs are only present for odd periods, ASs only for even periods ( that was an easy way to simulate it ). 
What I need is a cumulative running total by period:

A      B            YEAR     PERIOD AMOUNT_SUM
------ ------ ---------- ---------- ----------
BV     BS           2008          1        100
BV     AS           2008          2        200
BV     BS           2008          2        100
BV     AS           2008          3        200
BV     BS           2008          3        400
BV     AS           2008          4        600
BV     BS           2008          4        400
BV     AS           2008          5        600
BV     BS           2008          5        900
BV     AS           2008          6       1200
BV     BS           2008          6        900
BV     AS           2008          7       1200
BV     BS           2008          7       1600
...

But when I use the sum() analytic function

select A
, B
, year
, period
, sum(amount)
     over (partition by A, B
     order by A, B, PERIOD) AMOUNT_SUM
FROM test
WHERE YEAR = 2008
  and A = 'BV'
  AND PERIOD <= 7
ORDER BY PERIOD, A, B;

 I only get

A      B            YEAR     PERIOD AMOUNT_SUM
------ ------ ---------- ---------- ----------
BV     BS           2008          1        100
BV     AS           2008          2        200
BV     BS           2008          3        400
BV     AS           2008          4        600
BV     BS           2008          5        900
BV     AS           2008          6       1200
BV     BS           2008          7       1600
i.e. for periods where B does not have a value in the base table there is no cumulative sum. I need the prior period's cumulative sum carried forward.
Anybody have a bright idea how to accomplish that in a single SQL. The database is 9.2.0.6. Maybe wrapping the sql in another analytic function with a window looking back to the prior row. 



Regards

Wolfgang Breitling
Centrex Consulting Corporation
http://www.centrexcc.com

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