define function difference ($a) {
    element period {
      subtract-dates(date($a[2]),date($a[1])),
      attribute tstart {$a[1]},
      attribute tend {$a[2]}
    },
    if (exists (subsequence($a,3))) then (
      difference(subsequence($a,2))
    ) else ()
}
define function firstDate () {
  for $x in document('v-emps.xml')/employees/employee/title
  return $x
  sortby (@tstart ascending)
}
define function allRange () {
  for $x in document('v-emps.xml')/employees/employee/title[last()>1 and position() != last()]

  return $x
  sortby (date(@tend) ascending)
}
(difference((firstDate()[1]/@tstart, distinct-values(allRange()/@tend)))
sortby (int(text()) descending))[1]