Keeping a running total? (Redo)

----- Source XML
<xml>
 <factory x="A" capacity = "3" />
 <factory x="B" capacity= "5" />
  <factory x="C" capacity = "3" />
  <factory x="D" capacity = "2" />
  <factory x="E" capacity = "2" />
  ...etc...
</xml>

$Widget_quota = "8"
$Gadget_quota = "5"

    A        |   3
    B        |   5
<!--          *Click. Widget Quota has been reached. Switch to gadgets* -->
    C        |                       3
    D        |                       2
<!--          *Click. Omg. Gadget Quota has been reached. Switch to Excess* -->
    E         |                                             etc.
    F         |                                             etc.
    G         |                                             etc.
    H         |                                             etc.

----
Howto?

M.K. Suggested
http://www.biglist.com/lists/xsl-list/archives/200607/msg00191.html
but my question (Howto?) remains.