query: SELECT ?item ?group ?start 
    WHERE { 
      ?item p:P39 ?ps . 
      ?ps ps:P39 wd:Q21328626 ; pq:P2937 wd:Q56229998 .
      OPTIONAL { ?ps pq:P580 ?start }
      OPTIONAL { ?ps pq:P582 ?end }
      OPTIONAL { ?ps pq:P4100 ?group }
      FILTER(!BOUND(?end)