[O] Limiting Invoice Block

[Esben Stien]

Playing around with org-invoice, but can't figure out how to limit the
block to subtree.

Why is this not legal as it is with other blocks?:

#+BEGIN: invoice :maxlevel 2 :scope subtree :link t :tstart "<2012-09-03
 Thu 10:00>" :tend "<2012-09-06 Thu 12:00>"

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