aquantity.
Journal entries still require a positive @@ price, but now the sign is
set after parsing, rather than when converting in amountToCost.
The reason for this change is that, if we're going to perform arithmetic
on Amount with TotalCost, then the presence of aquantity=0 means that
amountToCost would render the total cost as 0, because signum 0 == 0.
This makes journal entries like the following impossible to balance:
2000-01-01
a 0 @@ 10 A
b -10 A
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