def within(self, lat, lon, radius):
self._geo = { "<STR_LIT>": { <EOL>"<STR_LIT>": [<EOL>factual.common.shared_filter_helpers.GeoScalar(lat), <EOL>factual.common.shared_filter_helpers.GeoScalar(lon), <EOL>], <EOL>"<STR_LIT>": radius <EOL>}<EOL>}<EOL>return self<EOL>
Apply a geo=$circle/$center filter. Radius is in meters. Subsequent within() 
calls replace earlier geo filters on this query.