To support the EvidenceVariable use for data exchange including both "simple structure" and expression-language-specific versions of characteristic definitions, and especially for the January connectathon with interests in sharing EvidenceVariable content for use with different execution engines, there is a need to allow both EvidenceVariable.characteristic.definitionExpression and one of the other EvidenceVariable.characteristic.definitionXXX elements to have compatible content. This is much simpler than requiring extensions or relatedArtifact use.

Proposed disposition: Change the invariant rule ("In a characteristic, at most one of these seven elements shall be used: definitionReference or definitionCanonical or definitionCodeableConcept or definitionExpression or definitionId or definitionByTypeAndValue or definitionByCombination") to remove "or definitionExpression" from this rule.  Add to the definitionExpression an invariant rule of: 'When another element provides a definition of the characteristic, the definitionExpression content SHALL match the definition (only adding technical concepts necessary for implementation) without changing the meaning.'