At connectathon we realized that the Coverage resource is rather ambiguous about where the member id should be stored, and Epic's version (storing it in Coverage.dependent, unless member and subscriber are the same person, then in Coverage.subscriber) didn't line up with Evernorth/eviCore (storing it as a Patient identifier).

We should update the CRD coverage profile to have a mandatory slice on Coverage.identifier of type "MB" (see http://hl7.org/fhir/R4/v2/0203/index.html) so it's clear to everyone where to store and retrieve the member number.