Problem: Implementers find the current approach to defining a characteristic confusing. It's not evidently clear that defByTypeAndValue and defByCombination are an alternative to definition[x]. and it's not clear what "defBy" means.

Currently EvidenceVariable.characteristic requires one of:

• definition[x]: definitionReference, definitionCanonical, definitionCodeableConcept, definitionExpression, or definitionId
• defByTypeAndValue
• defByCombination

What we want to do with EvidenceVariable.characteristic is to break out and rename the elements as:

• definitionReference
• definitionCanonical
• definitionCodeableConcept
• definitionExpression
• definitionId
• definitionByTypeAndValue
• definitionByCombination

And the rule for that is you can have at most one of those seven elements for each characteristic.

Comments and feedback welcome.