The logical design of an expression repository includes the entities required to hold the data relevant for an expression version. 

As illustrated in , an expression repository contains one or more expression versions.

Each expression version must contain the following data:

• Versioning data, to represent the time and substrate of the given version of the expression
• A Close-to-user form expression, to represent the expression as it was stated upon creation

Each expression version should contain the following data:

• An expression identifier, if this is required to uniquely identify the expression
• A Classifiable form expression, to enable the generation of the NNF expression
• A Necessary Normal Form expression, to support querying the expression

All SNOMED CT concepts referenced by expressions in the repository at a particular point in time should all be included, and active, in a specific versioned edition of SNOMED CT.

General expression repository design.

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