code: public void performClear(Transaction tx, Resource... contexts) {
          if(notNull(contexts)) {
              for (int i = 0; i < contexts.length; i++) {
                  if (notNull(contexts[i])) {
                      graphManager.delete(contexts[i].stringValue(), tx);
                  } else {
                      graphManager.delete(DEFAULT_GRAPH_URI, tx);
                  }
              }
          }else{
              graphManager.delete(DEFAULT_GRAPH_URI, tx);
          }
      }
docs: clears triples from named graph
  
  @param tx
  @param contexts