This is necessary for credit forms, because it is “just” an invoice with
a negative amount, usually the inverse of an invoice that has to be
rectified. This is a requirement for electronic invoicing too.
However, it is not the only use case: sometimes it is necessary to have
some lines with a negative quantity, even if the total is positive, to
emphasize discounts or to register expenses, like bank commissions in
autoinvoices.
Part of #61.
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