code: @Override
      public void startLearning() {
          if (hypothesis.isInitialized()) {
              throw new IllegalStateException();
          }
  
          TTTState<I, D> init = hypothesis.initialize();
          AbstractBaseDTNode<I, D> initNode = dtree.sift(init.getAccessSequence(), false);
          link(initNode, init);
          initializeState(init);
  
          closeTransitions();
      }
docs: /*
  Private helper methods.