How to create a Column that will "carry over" values

Question

I have the first 5 colums of the following table and I would like to create the last column (Carryover). The calculation for carryover (current row) is PRIOR_DAYS_OF_SUPPLY(current row)-DAYS_BTW_REFILL(current row)-carryover (prior row)
This is tricky since the value is dependent on the prior row value of the SAME column. I have tried the window functions but still cannot get it to work.
P_ID     RX_DATE     DAYS_OF_SUPPLY     PRIOR_DAYS_OF_SUPPLY     DAYS_BTW_REFILL     CARRYOVER
100     10/20/2005     30               0
100     11/28/2005     30     30     39     0
100     12/21/2005     30     30     23     7
100     1/13/2006     60     30     23     14
100     3/3/2006     30     60     49     25
100     4/14/2006     30     30     42     13
100     6/9/2006     30     30     56     0
100     7/7/2006     30     30     28     2
100     7/31/2006     30     30     24     8
200     10/20/2005     30               0
200     11/28/2005     30     30     30     0
200     12/21/2005     30     30     35     0
200     1/13/2006     60     30     25     5
200     3/3/2006     30     60     70     0
200     4/14/2006     45     30     32     0
200     6/9/2006     30     45     41     4
200     7/7/2006     30     30     30     4
200     7/31/2006     30     30     32     2
Any help would be greatly appreciated,
Sam

Answer 1

I may not be explaining my issue well enough. I believe that 10g has new funciontality that can solve this. Since I am in 9i, I would like to have something that does the same thing as the following 10g SQL:
create table foo (P_ID number(3) ,R_DT date , DOS number(3) ,
PDOS number(3), DBTWR number(3), CO number(3));
insert into foo values (100,to_date('10/20/2005','mm/dd/yyyy'),'30',0,null,null );
insert into foo values (100,to_date('11/28/2005','mm/dd/yyyy'),'30','30','39',0);
insert into foo values (100,to_date('12/21/2005','mm/dd/yyyy'),'30','30','23',7);
insert into foo values (100,to_date('01/13/2006','mm/dd/yyyy'),'60','30','23',14);
insert into foo values (100,to_date('03/3/2006','mm/dd/yyyy'),'30','60','49',25);
insert into foo values (100,to_date('04/14/2006','mm/dd/yyyy'),'30','30','42',13);
insert into foo values (100,to_date('06/9/2006','mm/dd/yyyy'),'30','30','56',0);
insert into foo values (100,to_date('07/7/2006','mm/dd/yyyy'),'30','30','28',2);
insert into foo values (100,to_date('07/31/2006','mm/dd/yyyy'),'30','30','24',8);
insert into foo values (200,to_date('10/20/2005','mm/dd/yyyy'),'30',0,null,null);
insert into foo values (200,to_date('11/28/2005','mm/dd/yyyy'),'30','30','30',0);
insert into foo values (200,to_date('12/21/2005','mm/dd/yyyy'),'30','30','35',0);
insert into foo values (200,to_date('01/13/2006','mm/dd/yyyy'),'60','30','25',5);
insert into foo values (200,to_date('03/3/2006','mm/dd/yyyy'),'30','60','70',0);
insert into foo values (200,to_date('04/14/2006','mm/dd/yyyy'),'45','30','32',0);
insert into foo values (200,to_date('06/9/2006','mm/dd/yyyy'),'30','45','41',4);
insert into foo values (200,to_date('07/7/2006','mm/dd/yyyy'),'30','30','30',4);
insert into foo values (200,to_date('07/31/2006','mm/dd/yyyy'),'30','30','32',2);
================
Quote:
select p_id,r_dt,dos,pdos,dbtwr,cry
from foo
model return all rows
main grr
partition by ( p_id)
dimension by (row_number() over
(partition by p_id order by r_dt ) as rn )
measures (r_dt,dos,pdos,dbtwr,co, 0 as cry )
ignore nav rules sequential order
cry[any]= case when (pdos[cv()]-dbtwr[cv()] +cry[cv()-1])
< 1 then 0
else pdos[cv()]-dbtwr[cv()]+cry[cv()-1]
end
)