referentialIdentifier
A referential identifier is any identifier or keyword identifier that can appear in a referential (name-lookup) context. This allows CQL keywords that are not reserved to be used as identifiers.
Examples
Plain identifier
Patient
Quoted identifier
"Measurement Period"
Keyword used as identifier (e.g., 'date' as a property name)
Obs.date
EBNF
referentialIdentifier : identifier | keywordIdentifier ;