question about how to update an item in a field in batch mode

[Peter Alcibiades]

Trying Sarah's suggestion first, but cannot seem to make it work:  

on mouseUp
  set the itemDelimiter to tab
  repeat for each line L in field "tSales"
    get lineOffset(item 1 to 2 of L, "tStock")
    if it > 0 then subtract 1 from item 5 of line it of field "tStock"
  end repeat
end mouseUp

It does nothing.  The reason being that variable it is always 0, though there 
should be a match for items 1 to 2.  If I make it just match on item 1, same 
thing.

Then if I take out the condition, something even stranger happens.  It writes 
as many zeros as there are items doing the match before item 1 of line 1 of 
tStoc.  (there are two items which should match in the sample file, and for 
some reason the only one it finds is the second one!)

But I can't understand why it doesn't work.  It doesn't seem to violate the 
rule about not changing a variable during repeat.

Seem to be making heavy weather out of this one.  On to Jeanne's suggestion 
now.

Peter