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The process of transforming an expression to a normal form is based on the description logic definitions of the concepts referenced by the expression. Using this approach, expressions that are authored, stored and/or communicated in a relatively informal close-to-user form are logically transformed into a common normalized form. In this normalized form it is possible to apply simple rules to test subsumption between expressions .

The simplest case of a valid close-to-user expression is a single conceptId, and the approach described can be applied to these simple precoordinated expressions, as well as to more complex expressions that include multiple conceptIds and refinements ( qualifiers ).

The approach to normalization may be applied to specific expressions but may also be extended to take account of contextual information derived from the information model in which the expression is situated. Therefore, the normal form may include SNOMED CT context information, even if this is not present in the initial SNOMED CT expression .

The algorithm extends earlier work on canonical forms as follow:

  • Normalizes fully defined values within definitions or expressions producing nested expressions that are fully normalized .
  • Merges refinements stated in an expression with definitional relationships present in the definitions of the concepts referenced by the expression:
    • The merge process takes account of refinements that may not be grouped or nested in a manner that precisely reflects the structure of a current (or future) concept definition;
    • This avoids the need to add, store and communicate potentially spurious detail from current definitions to the expression recorded by a user or software application.
  • Takes account of context rules including default context and a preliminary approach to moodCode mapping and handling of procedures with values (present in algorithm but not yet easily visible in test environment).
  • Supports subsumption tests that take account of finding specified with 410516002 |known absent| finding context.

illustrates an overview of the process of normalization of an expression . Subsequent sections describe the processes shown in this diagram.

Figure 57. Overview of expression normalization process