(:
Temporal Join. For each employee, show his/her title history
and his/her manager history.
:)
declare function local:overlap($a, $ts,$te) {

if (xs:date($a/@tstart) <= xs:date($ts) and
      xs:date($a/@tend) >= xs:date($te))
  then (
     element mgrno {
	attribute tstart{$ts},
	attribute tend{$te},
	$a/text()
     })

else if (xs:date($a/@tstart) <= xs:date($ts) and
      (xs:date($a/@tend)) >= xs:date($ts) and
      (xs:date($a/@tend)) <= xs:date($te))
  then (
     element mgrno {
	attribute tstart{$ts},
	attribute tend{$a/@tend},
	$a/text()
     })
     
 else if (($a/xs:date(@tend) >= xs:date($te)) and
      ($a/xs:date(@tstart) >= xs:date($ts)) and
      ($a/xs:date(@tstart) <= xs:date($te)))
  then (
     element mgrno {
	attribute tstart{$a/@tstart},
	attribute tend{$te},
	$a/text()
     }
  )
  else if (($a/xs:date(@tstart) >= xs:date($ts)) and
      ($a/xs:date(@tend) <= xs:date($te)))
  then (
     element mgrno {
	attribute tstart{$a/@tstart},
	attribute tend{$a/@tend},
	$a/text()
     }
  )
     else ()
};

declare function local:final() {
   for $r in doc("v-emps.xml")/employees/employee
     for $m in doc("v-depts.xml")/departments
     /department[deptno=$r/deptno]
     let $ov := local:overlap($r/deptno, $m/mgrno/@tstart, $m/mgrno/@tend)
   
     return (element emp{$r/empno, $r/title, $r/deptno,
            for $e_temp in $r/deptno
              for $m_temp in $m/mgrno 
                 let $ov := 
                   local:overlap($e_temp, $m_temp/@tstart, $m_temp/@tend) 
              where not (empty($ov)) return $m_temp})
      };
      
      let $f := local:final()
      return element all_emps{$f}