def _expand_type(res_type):
    """Expand res_type to a resource type suitable for use as a filter
    in the tag command
    """
    EXPANSION_MAP = {   # in alphabetical order
                        'igw'  :        'internet-gateway',
                        'inst' :        'instance',
                        'nacl' :        'network-acl',
                        'rtb'  :        'route-table',
                        'sg'   :        'security-group',
                        'snap' :        'snapshot',
                        'vol'  :        'volume',
                    }
    return EXPANSION_MAP.get(res_type, res_type)