one of the documented procedures in this installation of the ACS
vc_path_relative   path
What it does:
Returns the component of path relative to [acs_root_dir]. If path does not begin with [acs_root_dir] it is returned unchanged.
Defined in: /web/philip/tcl/vc-defs.tcl

Source code:

    set root [acs_root_dir]

    # verify that path starts with $root
    if { [string first $root $path] == 0 } {
	# it does, so trim out the leading part
	set path [string range $path [expr { [string length $root] + 1 }] end]

    return $path