This case-insensitive code system http://hl7.org/fhir/uv/saner/CodeSystem/MeasurePopulationSystem defines the following codes:

Code	Display	Definition
numerator-complement	Numerator Complement	Like a numerator, but expresses that part of the Denominator not present in the Numerator
duration	Duration	A Time Duration
duration-squared	Duration Squared	The square of the time duration, useful for identifying variance
supporting	Supporting	A population supporting computation, but not needed for reporting