We have found it useful to add extensions to the ParameterDefinition metadatatype for use in library.parameter.  These extensions carry additional information that is present in the CQL libraries but is not expressible in the base ParameterDefinition data type.  The extensions we added were the following:

• Element type, for types like List and Interval.  Value is a required Code of FHIRAllTypes
• CQL type, because the crosswalk between CQL and FHIRAllTypes is incomplete and lossy.  Value is a string whose value is the URI from the model info e.g. {http://hl7.org/fhir}Condition
• Access level.  Value is a required Code of public|private.  Consider a code system here in case we want to extend to include other access modifier types in the future
• Default value, for in parameters declared with the CQL parameter keyword.  Expressed using various FHIR elements e.g. decimal, string, period, etc.
• Default value CQL type, because the crosswalk is lossy.  Expressed the same way as CQL type.

As an implementer of CRMI, our receiving system which imports these measures into our computation platform needs this information to be able to correctly pass values in and receive values out of libraries.  We have been adding these as extensions with an ncqa.org URL, but we would prefer these to be CRMI canonical extension URIs.