define function calAverage ($instance) {
  for $x in document('v-emps.xml')/employees/employee/salary
  let $value := int($x/text())
  where
    $x/date(@tstart) <= date($instance) and
    $x/date(@tend) > date($instance)
  return $value
}

define function allDate() {
  for $x in document('v-emps.xml')/employees/employee/salary
  return $x
  sortby (@tstart ascending)
}

for $snapshot in distinct-values(allDate()/@tstart)
return 
element average {
  avg(calAverage($snapshot)),
  attribute tstart{$snapshot}
}