code: def register_element_cls(tag, cls):
      
      nspfx, tagroot = tag.split(':')
      namespace = element_class_lookup.get_namespace(nsmap[nspfx])
      namespace[tagroot] = cls
docs: Register *cls* to be constructed when the oxml parser encounters an
      element with matching *tag*. *tag* is a string of the form
      ``nspfx:tagroot``, e.g. ``'w:document'``.