code: BeatGrid getBeatGrid(int rekordboxId, SlotReference slot, Client client)
              throws IOException {
          Message response = client.simpleRequest(Message.KnownType.BEAT_GRID_REQ, null,
                  client.buildRMST(Message.MenuIdentifier.DATA, slot.slot), new NumberField(rekordboxId));
          if (response.knownType == Message.KnownType.BEAT_GRID) {
              return new BeatGrid(new DataReference(slot, rekordboxId), response);
          }
          logger.error("Unexpected response type when requesting beat grid: {}", response);
          return null;
      }
docs: Requests the beat grid for a specific track ID, given a connection to a player that has already been set up.
  
  @param rekordboxId the track of interest
  @param slot identifies the media slot we are querying
  @param client the dbserver client that is communicating with the appropriate player
  
  @return the retrieved beat grid, or {@code null} if there was none available
  
  @throws IOException if there is a communication problem