How to understand my expression that compute my stock amount ?

[paulyeo11]

Hi All

I have this expression few year back from Sunny , it work fine. But i don't understand how this exprssion compute the right result.
Hope some one can share with me.

(RangeSum(Above(RangeSum
(Above(
Sum
(
{
<
year, month, sTOCK={sTOCK}
>
}
[Amount]), 0, RowNo()))),
-
Sum(
{
<
sTOCK={sTOCK}>
}
TOTAL Aggr(Sum(
{
<
year, month, sTOCK={sTOCK}
>
}
[Amount]), YearMonth
))))

[paulyeo11]

Hi All

Enclosed my QVW