[SS] 30Nov2004: The following commands are basic set operations for Tcl lists.
They are part of a larger library for functional programming in Tcl that's possible
to find at http://wiki.hping.org/133.
The proposed commands try to run in O(M+N) time complexity,
and to don't mess with the order of the elements when possible.


 proc lintersect {a b} {
     foreach e $a {
 	set x($e) {}
     }
     set result {}
     foreach e $b {
 	if {[info exists x($e)]} {
 	    lappend result $e
 	}
     }
     return $result
 }
 
 proc lunion {a b} {
     foreach e $a {
 	set x($e) {}
     }
     foreach e $b {
 	if {![info exists x($e)]} {
 	    lappend a $e
 	}
     }
     return $a
 }
 
 proc ldifference {a b} {
     foreach e $b {
 	set x($e) {}
     }
     set result {}
     foreach e $a {
 	if {![info exists x($e)]} {
 	    lappend result $e
 	}
     }
     return $result
 }
 
 proc in {list element} {
     expr {[lsearch -exact $list $element] != -1}
 }