code: @Override
      protected void fireKeyCompleted(String keyId, List<Proposition> data)
              throws DataSourceReadException {
          assert this.handler != null : "handler cannot be null";
          this.handler.handle(keyId, data);
      }
docs: Called by {@link MultiplexingDataStreamingEventIterator} when a key id's
  propositions have been completely assembled.
  
  @param keyId a key id {@link String}.
  @param data
  @throws DataSourceReadException