Consider the following definitions:
Entity > Animal, Person. Animal > Bird, Mammal. Mammal > Dog, Cat, Mouse. Person > Student, Employee. Employee > Professor, TeachingAssistant. Professor > FullProfessor, AssistantProfessor.
This tells us, among other things, that Animal is a subtype of Entity; that Bird and Mammal are both subtypes of Animal; that Dog, Cat, and Mouse are all subtypes of Mammal; and that FullProfessor and AssistantProfessor are subtypes of Professor.
This may be drawn as in the following type hierarchy:
Notice that although Animal and Person are both subtypes of Entity, this does not mean that there can be no definitions in between. For example, both could be included in the common supertype Animate. We just haven't specified this in the above example.
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