Consider new datatype for instantiatesCanonical/instantiatesUri

There are a number of resources which contain the following two elements:

• instantiatesCanonical
• instantiatesUri

This occurs 16 times (across 14 resources).  Since this seems to be a well-established pattern, and since these elements are similar in purpose, it might be beneficial to formalize this construct.

Unfortunately, instantiates[x] cannot be used because the cardinality is 0..* on many of these (and perhaps some instances should use both kinds).

This is why we should consider creating a new datatype to represent this pairing, similar to CodeableReference.

One question that arises is if it ever make sense to have both a canonical and uri representation paired (indicating different representations of the same thing)?

• If yes (pairing a canonical and uri could make sense), then this is much like CodeableReference - so there is precedent to introduce a type like this.
• If no (it should always be a canonical or a uri), then the datatype would either have two elements w/ an invariant OR would have a single target[x] element (canonical | uri). Introducing such a thing likely requires more thought.

If this has been discussed in the past, I apologize for bringing it up again!  But if not, it is probably worthy of some discussion.

Here is a listing of where we find this pattern in the FHIR R5 Ballot spec:

0..* CarePlan.instantiatesCanonical
0..* CarePlan.instantiatesUri
0..* CarePlan.activity.plannedActivityDetail.instantiatesCanonical
0..* CarePlan.activity.plannedActivityDetail.instantiatesUri
0..* Communication.instantiatesCanonical
0..* Communication.instantiatesUri
0..1 Contract.instantiatesCanonical
0..1 Contract.instantiatesUri
0..* DeviceRequest.instantiatesCanonical
0..* DeviceRequest.instantiatesUri
0..* FamilyMemberHistory.instantiatesCanonical
0..* FamilyMemberHistory.instantiatesUri
0..1 GenomicStudy.instantiatesCanonical
0..1 GenomicStudy.instantiatesUri
0..1 GenomicStudy.analysis.instantiatesCanonical
0..1 GenomicStudy.analysis.instantiatesUri
0..* NutritionIntake.instantiatesCanonical
0..* NutritionIntake.instantiatesUri
0..* NutritionOrder.instantiatesCanonical
0..* NutritionOrder.instantiatesUri
0..* Procedure.instantiatesCanonical
0..* Procedure.instantiatesUri
0..* RequestGroup.instantiatesCanonical
0..* RequestGroup.instantiatesUri
0..* RequestOrchestration.instantiatesCanonical
0..* RequestOrchestration.instantiatesUri
0..* ServiceRequest.instantiatesCanonical
0..* ServiceRequest.instantiatesUri
0..1 Task.instantiatesCanonical
0..1 Task.instantiatesUri
0..1 Transport.instantiatesCanonical
0..1 Transport.instantiatesUri