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A terminology server should support selective retrieval by facilitating testing of expressions against query predicates .

Using expressions is the most common approach for supporting postcoordination .

Example use cases:

  1. Users define a postcoordinated expression, the server verifies if the expression is an exact match with an existing precoordinated concept or if it will be stored in a postcoordinated expressions repository as a new or existing expression .
  2. For a epidemiological purposes a predicate expression is created as a query. The predicate expression is matched against candidate expressions and concept references stored in the clinical record, to retrieve all content equivalent or subsumed by the predicate expression .

To facilitate complete and accurate retrieval of precoordinated and postcoordinated expressions from clinical records or other resources it is necessary to compare an expression in a record with a query predicate. This comparison needs to determine if the candidate expression is subsumed by the predicate.

The same meaning can be represented in different postcoordinated expressions and to facilitate comparison expressions with the same meaning can be converted to a common normal form. This section describes the process of normalization and the approach to testing for subsumption between the resulting normal form expressions .


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