code: def register_element_cls(nsptagname, cls):
      
      nsptag = NamespacePrefixedTag(nsptagname)
      namespace = element_class_lookup.get_namespace(nsptag.nsuri)
      namespace[nsptag.local_part] = cls
docs: 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'``.