let … in statement
The statement let <VAR> in <EXPRESSION> assigns the results of a stream valued expression <EXPRESSION> to the variable <VAR>,
one at a time.
The expression can only be a call to a function that returns a stream of answers.
If each tuple in the stream contains more than one value, multiple variables must be specified of the left hand side of the assignment.
let $year, $company in past-employments($person);