one of the documented procedures in this installation of the ACS
What it does:
Checks to see if a user is logged in. If not, redirects to /register/index to require the user to register. When registration is complete, the user will return to the current location. All variables in ns_getform (both posts and gets) will be maintained. Note that this will return out of its caller so that the caller need not explicitly call "return". Returns the user id if login was succesful.
Defined in: /web/philip/packages/acs-core/security-procs.tcl

Source code:

    set user_id [ad_verify_and_get_user_id]
    if { $user_id != 0 } {
	# user is in fact logged in, terminate
	return $user_id