OpenBGPD as route server - correct filter syntax ?

Hi,

I'm trying to figure out the most suitable config params transform OpenBGPD into a route server.

So far I have :
route-collector yes
transparent-as yes

But my concern is more in the area of suitable filters to prevent loops.

I'm thinking I need something along the lines of :

allow to any peer-as != neighbor-as

But any variation of the above that I try always ends up with a "bgpd -n -f" throwing a syntax error.

Ideas most welcome !

Thanks

Bob