List the common prime minister of the Robert Ray and graham richardson ?
SELECT DISTINCT ?uri WHERE { <http://dbpedia.org/resource/Robert_Ray_(Australian_politician)> <http://dbpedia.org/ontology/primeMinister> ?uri. <http://dbpedia.org/resource/Graham_Richardson> <http://dbpedia.org/ontology/primeMinister> ?uri . }
Robert_Ray_(Australian_politician) Graham_Richardson