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:

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