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Figure 6: Metaschema fragment for main ORM object types and fact types
Figure 6 shows another fragment of the ORM metschema. An object type either is
primitive or is a subtype. A primitive object type is independent if its instances can exist
independently of playing any role in a fact type. Subtyping in ORM allows multiple inheri-
tance (e.g., AsianWoman may be a subtype of both AsianPerson and Woman, each of which
is a subtype of Person). Subtypehood is acyclic and intransitive.
An object type is also either non-lexical (entity type) or lexical (value type). A nested
entity type is an entity type constructed by objectifying a fact type. For example, the fact
type ObjectType is a subtype of ObjectType has been objectifi ed as SubtypeConnection.
Treating the nested object type as distinct from its source fact type allows a single inheri-
tance implementation of this part of the type hierarchy. Alternatively, a nested entity type
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