The SDC IG has defined and pre-adopted a number of FHIRPath variables and functions that SHOULD be defined as part of the core FHIR spec FHIRPath extensions.  They can be found here: https://build.fhir.org/ig/HL7/sdc/expressions.html#fhirpath

Specifically, we should define the %questionnaire, %qitem, answers(), and ordinal() variables and functions