FHIR Filter-operator Page
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The is_a relationship is a transitive relationship. It is not clear from this definition of the operators whether (often implicit) transitive is_a relationships are considered. For example, is A is_a B, and B is_a C, then does the is_not_a operator match on A, C or not? [I would hope not!]
This is particularly significant when considering SNOMED CT and its derivatives. As a special case, the current V2 series of the Australian Medicines Terminology (AMT) is distributed in so-called Stated Form rather than Distribution Normal Form which means that many is_a relationships are actually omitted (and this is a slightly different case from the simple transitive closure case mentioned above).