ral Aggregate. For each department show 
the history of employee count in the department over time.
:)

declare function local:init($r) {
let $table := (element department{($r), 
element time{xs:date($r/@tstart)}, element change{"+"}},
element department{($r), element time{xs:date($r/@tend)}, 
element change{"-"}} )
return $table
};

declare function local:sort($xml) {
for $e in $xml
order by $e/time ascending
return ($e)
};

declare function local:embrace() {
for $r in doc("v-emps.xml")/employees/employee/deptno
let $xml := local:init($r)
return $xml
};


declare function local:overlap($time, $d) {
for $r in doc("v-emps.xml")/employees/employee/deptno[text()/$d/text()]
where (xs:date($r/@tstart) <= xs:date($time) and xs:date($r/@tend) >= xs:date($time))
return $r
};


declare function local:emp_count($e) {
if ($e/change/text() = "+")
then (
let $num_overlap := count(local:overlap($e/time, $e/deptno)) 
return $num_overlap
)
else if ($e/change/text() = "-")  then (
let $num_overlap := (count(local:overlap($e/time, $e/deptno))-1)
return $num_overlap
)
else ()
};

declare function local:final() {
let $big_xml := local:embrace()
let $sorted_deps := local:sort($big_xml)
for $e in $sorted_deps
order by $e/deptno
return element dept{($e/deptno), element emp_count{local:emp_count($e)}}

};

let $f := local:final()
order by $f/dept/deptno ascending
return element data{$f}