code:     public XMSSMTPrivateKeyParameters getNextKey()
      {
          BDSStateMap newState = new BDSStateMap(bdsState, params, this.getIndex(), publicSeed, secretKeySeed);
  
          return new XMSSMTPrivateKeyParameters.Builder(params).withIndex(index + 1)
              .withSecretKeySeed(secretKeySeed).withSecretKeyPRF(secretKeyPRF)
              .withPublicSeed(publicSeed).withRoot(root)
              .withBDSState(newState).build();
      }
  