alias
A local name bound to a query source within a query. The alias is an identifier that shadows the source expression and is used to refer to individual elements in subsequent query clauses.
Examples
Single-letter alias (conventional)
[Condition] C where C.clinicalStatus ~ "Active"
Descriptive alias
[Encounter: "Inpatient"] Enc where Enc.period during "Measurement Period"
EBNF
alias : identifier ;