The VSD expects that a value set resource would support describing an expression to be used to generate the expansion that uses a different syntax then the built in FHIR compose include, exclude, vs reference, and filter symantics. While there is a syntax based on MIF in VSD, VSD also supports identifying a different syntax and then a text field to embed the text representation of that syntax. It is understood that if a syntax is restricted, and described, within a single code system, the filter element can be used to capture the expression. The problem is there are syntaxes that are not described by the code system and instead are independent, such as Apelon TQL, or SQL, or OWL, etc. VSD supports identifying what syntax is used through the CLDSyntaxReference element. This needs to be avaialble in FHIR.

The need for this has already resulted in the CDC Opioid Prescribing Support IG (http://build.fhir.org/ig/cqframework/opioid-cds/index.html) creating an extension to value set resource (http://build.fhir.org/ig/cqframework/opioid-cds/cdc-valueset.html) that has a specific extension providing this: http://build.fhir.org/ig/cqframework/opioid-cds/cdc-valueset-query.html

There is an even better solution using R4 "Expression dataype" http://build.fhir.org/metadatatypes.html#Expression. Using this, a solution would be to add an extension to value set that has a single element "Expression" with the noted datatype. 0..1 (although the description text for Expression.description should use the word "expression" instead of condition.