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'``.

def register_element_cls(tag, cls):
    """
    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'``.
    """
    nspfx, tagroot = tag.split(':')
    namespace = element_class_lookup.get_namespace(nsmap[nspfx])
    namespace[tagroot] = cls