Abstract base of the three kinds of properties (excluding ConstraintProperty) that can be typed by a Block: PartProperty, SharedProperty, ReferenceProperty
SysML1.0:
A Block that has composite aggregation is classifed as a part property. A property typed by a Block that does not have composite aggregation is classified as a reference property.
MD SysML distinguishes between a SharedProperty and a true ReferenceProperty which both extend AbstractReferenceProperty.
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