letClauseItem
A single named binding within a let clause. The identifier names the binding and the expression provides its value, which is evaluated once and reused wherever the name appears in the query.
Examples
Bind an interval to a name
from [Encounter] E let period: E.period
Bind a computed value
from [Observation] O let score: O.value as Quantity
EBNF
letClauseItem : identifier ':' expression ;