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Extended Deep Taxonomy
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See http://ruleml.org/WellnessRules/files/WellnessRulesN3-2009-11-10.pdf

It is taking the depth as the number of elements in the classes.
It is closer to reality e.g. an ontology with 300000 classes (like SNOMED-CT)
and 100000 individuals belonging to those classes (like patients having
observations, diseases, treatments, ...).
