Regarding CMS68 Documentation of Current Medications in the Medical RecordFHIR:  Denominator Exception (Medications not documented in the Medical Reason)

Current attempt of translation into FHIR: 

"Qualifying Encounter during Measurement Period" QualifyingEncounter
with [Task: "Documentation of current medications (procedure)"] MedicationsNotDocumented
such that MedicationsNotDocumented.authoredOn during QualifyingEncounter.period
and MedicationsNotDocumented.reasonCode in "Medical Reason"
and MedicationsNotDocumented.status = 'completed'

The current logic includes procedure, performed union intervention, performed but we are thinking task is more appropriate. However, a negation rationale is included for the denominator exception and currently negation does not exists for Task. If Task is used how do we go about requesting a negation extension for the Task resource?  

Please advise.