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Ian McNicoll suggested an alternative whereby no new classes at all are added, but the CLUSTER class is given an extra attribute is_clinical_data_group: Boolean, to enable the same effect as a CLINICAL_DATA_GROUP_CLASS. Note that this would make clinical data group archetypes and nodes less visible in current tooling because the class is the basis for node visualisation (since icon, editing actions, context menu are based on class name).
Alternative #4
The simplest UML model that can enforce the following:
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