function should return a string that will be
    used as the main identifier for the module. Most of the time, this should be
    the absolute path to the module, but it can be any unique key that
    identifies the module such as an absolute URL. It is this key that is used
    to determine if a module has been loaded already.  This mechanism lets
    <code class="mendoza-code">./mymod</code> and <code class="mendoza-code">mymod</code> refer to the same module even though they
    are different names passe