In the Eligibility Criteria Connectathon Track, we needed to model a characteristic defined as chronic liver disease (Except for NAFLD).  To avoid creation of a value set (which would involve 74 codes by deleting the NAFLD code from the value set for chronic liver disease) or using an expression language, we attempted to use EvidenceVariable.characteristic.defByCombination but found simple combinations of "Condition = chronic liver disease" and "Condition != NAFLD" would fail for a patient who has both NAFLD and another chronic liver disease.

Adding one more code to the CharacteristicCombination code set was modeled as the simplest solution with an example at https://fevir.net/resources/EvidenceVariable/32960

The suggested code "except-subset" could be added with display "Except Subset" and definition "Combine by expressing a characteristic including multiple values and one or more characteristics that are a subset of those values to be excluded. Use of this code is combined with 1 and only 1 characteristic with exclude=false, and 1 or more characteristics with exclude=true that are limited to subsets of values contained within the included characteristic."