This is an example of how to do a calculation on a calculation.
You should regard this an example in development.  

How it works

This example contains methods in three places, two change value
methods and a custom method which the calculated field calls.

What the two changevalue methods do is calculate the difference
between the posted value for the current record, and the current
value.  For example, say you have qty of 2 and selling price of 7
posted in the table, and you've changed qty to 3 and price to 6 in
the current record. That means there is a difference of 4 (6*3)-
(7*2).  This value gets placed in the variable num in the var window
associated with the lineitem table object.

What the custom method (doit) does is scan the posted values in the
table, do a calculation based on them, then add that calculation to
the value calculated by the last changevalue method and place that
result where the result goes.  Finally, the custom method returns one.
