one of the documented procedures in this installation of the ACS
What it does:
Redirects user to /register/index to require the user to register. When registration is complete, the user will be returned to the current location. All variables in ns_getform (both posts and gets) will be maintained.
Defined in: /web/philip/packages/acs-core/security-procs.tcl

Source code:

    set form [ns_getform] 
    set url_args ""
    # note that there is no built-in function that will change
    # posted variables to url variables, so we write our own
    if ![empty_string_p $form] {
	set form_size [ns_set size $form]
	set form_counter_i 0
	while { $form_counter_i<$form_size } {
	    if {[string compare $form_counter_i "0"] == 0} {
		append url_args "?"
	    } else {
		append url_args "&"
	    append url_args "[ns_set key $form $form_counter_i]=[ns_urlencode [ns_set value $form $form_counter_i]]"
	    incr form_counter_i
    ad_returnredirect "/register/?return_url=[ns_urlencode [ns_conn url]$url_args]"