Robert P. J. Day schreef:
>    does your solution allow for the possibility of different invoices
>  of equal amounts?  i would be reluctant to use the word "subset" in a
>  context where you can have more than one element with the same value.

I think it handles different invoices of equal amounts correctly.
I agree with you though that the term subset may not be the best name in 
this context because of those duplicates.

cheers,
Ozz
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