"ERROR: 42883: could not identify an ordering operator for type polygon"

Hi,

I'm trying to setup Bucardo to do a master-master replication between two PostgreSQL servers.  However, it looks like
whenit tries to execute a query involving DISTINCT and a column with a data type of "polygon", the following error is
thrownby PostgreSQL: 

ERROR:  42883: could not identify an ordering operator for type polygon

I assume a default operator class for polygon needs to be defined using "CREATE OPERATOR CLASS"
(http://www.postgresql.org/docs/8.1/static/sql-createopclass.html). Does anyone have an example of an appropriate
operatorclass definition for this data type? 

Thanks,
Lawrence