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Introduction

A discussion paper from the Content Managers Advisory Group, entitled Discussion Paper - Allowance of Extensions to Modify Core Content has been submitted to SNOMED International management to resolve issues surrounding the interpretation of what national extensions are allowed to modify in the International release of SNOMED CT.  The discussion paper outlines a number of proposed clarifications to the interpretation of what is allowed and details the requirements for those changes.  This response proposes a series of policy statements that explicitly state what can and cannot be done to content of the International release by national extensions.

While this response is primality focused on the needs of national extensions, the general policies discussed here will apply to all extensions, regardless if they are directly dependent on the International release or another edition.  

Background

The primary concern expressed is the interpretation of Clause 4.1 of the SNOMED CT® AFFILIATE LICENSE AGREEMENT, which states: 

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The current interpretation of Clause 4.1 with regards to creation of new concepts is that the new extension concepts would necessarily be leaf nodes to the existing International releasebe be subtypes of an international concept, and not be a supertype of an international concept following classification (if the extension does perform classification of its content).  This type of content, by its very nature would have no impact on the international content, as it would be distal in the hierarchy to all international content.  National extensions have found that, due to gaps in the current International release, it is often necessary to insert intermediate (i.e. non-leaf) concepts into the terminology to meet user requests as well as to provide additional navigational support.  These intermediate concepts may be either primitive or fully-defined.

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1. The addition of the concept does not change the transitive closure for the International Release, except to add additional rows that include the new intermediate concept). The transitive closure of the national edition must be a superset of the transitive closure of the international edition. This necessitates the retention of all transitive IS A relationships from the international lease.
2. No stated or inferred defining attribute of core relationship of international content are is inactivated in the extension.
3. Inferred |is a| relationships should only be inactivated due to redundancy, but should still represent a valid relationship in the transitive closure. See "Retire (redundant) IS A relationships (not necessarily inferred)". 
4. No stated IS A relationships from a international concept to an extension concept is allowed.
5. Additional inferred defining relationships to not do not create crossovers (i.e. subtypes with attribute values that are supertypes of the values represented by the same attribute of the parent)
6. Additional inferred IS A relationships of subtypes do not represent a change in meaning as represented by the Fully Specified Name. 
7. The intermediate All extension concepts that are the supertype of an international concept, following classification, must be submitted for promotion to the international release as soon as possible..

Fully define concepts they create

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1. In cases where the classification results in an inferred IS A relationship from the core to become transitive(i.e. redundant). This In cases where the classification results in an inferred |is a| relationship from the core to become transitive (i.e. redundant), this redundant inferred relationship from the international release must be reactivated prior to the release.  I.e. transitive relationships may exist in a national edition as long as they originate from a different module from the extensioncore should not be inactivated, but should remain redundant in the release.
   

N.B. This policy has implications on the 'Expression Constraint Language's "childOf" and "parentOf" operators (which return the direct children/parents based on non-redundant |is a| relationships). Rather than relying on the inferred relationships in a release to determine the results of a childOf or parentOf constraint, a pre-processing step would be required to remove the redundant inferred relationships from the release.

State additional IS A relationships against core (international) concepts

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The incomplete modeling of international content resulting in a plethora of primitive content is well-recognized.  However, correction of these deficiencies within a national extension can lead to a number of unintended consequences including: 1) the impact on the classification process and the potential change in meaning of descendant concepts (ie.eg. loss of subtypes) 2) eventual remodeling of the primitive concept in the International release could lead to conflicts between the national edition and the International release.

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1. When needed to support national extension requirements, the addition of defining |non- IS A| relationships to existing primitive international content is allowed, providing:
   1. the additions are consistent with the current International Machine readable concept model
   2. the additions are submitted to SNOMED International as soon as possible for inclusion in the International release

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