Add a way to specify invariants in a definition that are applicable to the resource it defines.

Such as: population[1].count >= population[2].count to express a constraint (business rule in the definition) that the value of population[1].count in MeasureReport cannot exceed the value of population[2].count (the total number of beds must exceed or equal to the number of beds in use).  This may likely be some form of expression, e.g., FHIRPath or CQL that could be evaluated in the context of the defined resource type.

This would allow developers of definitions to create definition resources without having to create profiles just to get some level of validation support.