one of the documented procedures in this installation of the ACS
export_url_vars   args
What it does:
Returns a string of key=value pairs suitable for inclusion in a URL; you can pass it any number of variables as arguments. If any are defined in the caller's environment, they are included. See also export_entire_form_as_url_vars
Defined in: /web/philip/packages/acs-core/utilities-procs.tcl

Source code:

    set params {} 
    foreach var $args { 
	upvar 1 $var value 
	if { [info exists value] } {
            lappend params "$var=[ns_urlencode $value]" 
    return [join $params "&"]