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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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1. The RF2 distribution must not be modified, beyond appending additional rows. Overwriting data in the distribution files - such that an extension violates the append only model is not allowed (editor's addition). Additionally, the full history provided by the international release must provided, i.e. extensions may not omit anything;

2. Core International components must not be modified.
   Overriding the international content through either:

• the addition of new versions for core international components within extensions; or

• the addition of relationships to core international concepts and changing their DL definition.

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1. Create new concepts.

2. Fully define concepts they create.

3. Classify terminology extensions.

4. State additional IS A relationships against core (international) concepts.

5. Retire (redundant) IS A relationships (not necessarily stated).

6. Add additional defining (non-IS A) relationships to primitive core international concepts.

7. Retire content considered "inappropriate" - concepts, descriptions or relationships

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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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The creation of intermediate concepts necessarily results in changes to the relationships in the core international terminology.  These changes can be of almost any type including: assignment of new parent(s), retirement of inferred or stated parents, inheritance of new/additional defining relationships, removal of inferred relationships by the classifier, etc. There are (at least) 2 ways in which an extension concept can become an "intermediate" concept in an edition:

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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 coreinternational 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 core 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

Full definition of intermediate concepts will in many cases result in the reassignment of core international relationship due to inferences assigned from the classifier (see next section).  Likewise, any subtypes that are inferred under this concept may have their inferred defintions changed (depending on the modeling pattern applied to the extension concept). 

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The ability to classify extensions is an essential part of leveraging the description logics represented in the International Release for the proper maintenance of extension content.  However, in some cases the results of classification may change the inferred relationships of existing core international content.  

Proposed policy

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 core 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 extensionshould 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

Missing stated IS A relationships for core international content is usually the result of the existence of intermediate primitive concepts that editors are unaware of.  These intermediate primitive concepts are under review, but the volume makes it unlikely that all will be identified and resolved in the very near future.  Identification of these missing stated relationships by national extensions provides an opportunity for incremental improvement of the core international terminology.   

Proposed policy

1. Addition of new stated IS A relationships to core international content is not allowed.
2. Where a needed IS A relationship has been identified, a request to SNOMED International for addition of this relationship should be submitted as soon as possible.  These will be handled as high priority by the editorial staff. 

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1. Inactivation of International release stated IS A relationships is not allowed. 
2. Relationships identified as redundant and impacting extension content should be reported to SNOMED Int'l as soon as possible.   
3. Where retirement of stated relationships are needed, a request should be made to SNOMED International to correct the error.
   

Add additional defining (non-IS A) relationships to primitive

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international concepts

The incomplete modeling of core 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.

It is recognized that there may be cases where proper structuring of a national extension requires the full definition of currently primitive core international content and that the immediate needs of an extension do not coincide with the timing of International releases. The potential dangers of content divergence in national extensions by allowing definition of core international primitive content must be recognized by extension managers prior to undertaking such an effort. It must also be recognized that allowing the definition of international concepts to be changed in an extension makes cross-border, cross-organisation exchange/management/querying of health records more difficult than it already is.

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1. When needed to support national extension requirements, the addition of defining |non- IS A| relationships to existing primitive core 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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It is recognized that there are components within the International release that are erroneous. These is a current issue tracker IHTSDO-724 - Retire inappropriate synonyms, that will address the presence of inappropriate synonyms. The existence of other components deemed inappropriate by a national extension are often context or realm dependent.  The inactivation of international release components within a national extension can result in substantial deviation from the International release and thus an alternative representation of SNOMED core international content.  The integrity of SNOMED as an internationally consistent terminology would thus be compromised. 

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