def network_ref_modifier_key(tags):
    """ Convert relation tags to network, ref keys.
    
        Used by ParserOSM.parse().
    """
    if 'network' not in tags:
        return None
    if 'ref' not in tags:
        return None
    if not tags['network'] or not tags['ref']:
        return None
    return tags['network'], tags['ref'], tags.get('modifier', '')
