PREFIX  o:    <http://example>
# Original query
SELECT count(DISTINCT  *)
WHERE
   { ?d1      o:hasCode       "15" ;
              o:hasInitiator  ?writer .
     ?writer  o:hasUserName   ?writerName .
     ?d2      o:hasCode       "21" ;
              o:hasInitiator  ?reader .
     ?reader  o:hasUserName   ?readerName .
     FILTER ( ?d1 = ?d2 && ?writer != ?reader )
   }