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Concept B is a specialization of concept A if and only if:
every instance of concept B is also an instance of concept A; and
there are instances of concept A which are not instances of concept B.
Equivalently, B is a specialization of A iff A is a generalization of B.
For instance, 'Bird' is a specialization of 'Animal' because every bird
is an animal, and there are animals which are not birds (humans, for
instance).