nly - but not exclusively - for correlating with external data sources. 
Strategies for de-identification need to consider whether re-identification with the source system is a problem, 
in which case ids will need to be modified - and consistently across the resource set being de-identified. 
External identifiers will mostly need to be removed, but even then, where they are used for internal references
within the resource set, they'll need to be changed consistently.
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Then, there is the question of 