Encapsulates a symbolic quantity:

• May be typed directly by a *Quantity.
• Has a dimension:Dimension, via *DimensionedType
• Must never be assigned a defaultValue:ValueSpecification[0..1], since that assumes an assigned Unit (noting that even DimensionLess quantities may have numerical units).
• A Slot corresponding to a QuantityProperty may never be assigned a value:ValueSpecification[*], since that assumes an assigned Unit.

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