PREFIX geo: <http://www.opengis.net/ont/geosparql#>
PREFIX geof: <http://www.opengis.net/def/function/geosparql/>

SELECT ?within
WHERE { <http://example.org/brussels> geo:asWKT ?coord1 .
        <http://example.org/colorado> geo:asWKT ?coord2 .
        BIND ( geof:sfWithin(?coord1, ?coord2) as ?within )
}