Register *cls* to be constructed when the oxml parser encounters an
    element having name *nsptag_name*. *nsptag_name* is a string of the form
    ``nspfx:tagroot``, e.g. ``'w:document'``.

def register_element_cls(nsptagname, cls):
    """
    Register *cls* to be constructed when the oxml parser encounters an
    element having name *nsptag_name*. *nsptag_name* is a string of the form
    ``nspfx:tagroot``, e.g. ``'w:document'``.
    """
    nsptag = NamespacePrefixedTag(nsptagname)
    namespace = element_class_lookup.get_namespace(nsptag.nsuri)
    namespace[nsptag.local_part] = cls