The value the enumeration admits
Optional ReadonlyinAllowed value (closed enumeration).
When specified, values must be equal to it. The domain holds two values, so the constraint states the single admitted one rather than a list, which would either readmit the whole domain or close it to nothing; the boolean factory takes it as a bare value too. Closing the domain also narrows the value the shape describes to the stated one, wherever it is stated precisely enough to be told apart; a value only known to be a boolean leaves the shape admitting both.
Inheritance — must be strictly equal when both parent and child define it; otherwise the single defined value carries through. A mismatch is reported as an error.
Constraints accepted by the boolean shape factory.