a subclass of Expression; represents the (aggregate) instance that results from extracting from an aggregate Expression operand the corresponding collection of member instances that satisfy a given select-condition. Every Query is also the LocalScope for the Variable that designates members of the aggregate value in the select-condition.

represents the operand Expression whose result is the aggregate value from which members will be extracted by the Query operation.

represents the Boolean Expression used to evaluate members of the aggregate value. When this Expression returns True for a given member, that member is included in the Query result. Note: This role of Expression is a "thunk" the elaboration of the Query causes this expression to be evaluated separately for each member of the aggregate value.