Amount Validation
Whether an amount can be bought for a given ecode, and when it cannot, why.
A boolean would answer only whether to enable the buy button; a purchase form also has to tell the member what to change, which is why each refusal carries the bound or the set that refused it.
Inheritors
Types
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Above a range ecode's ceiling, including positive infinity.
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Below a range ecode's floor.
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Not one of a fixed-denomination ecode's amounts. Positive infinity is reported this way for a fixed ecode.
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Zero, negative or not a number (NaN, negative infinity) - never a purchase, whatever the ecode allows.
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The amount can be submitted for checkout.