code: @Override
      protected Deque<Step> initialValue() {
          Deque<Step> queue = new LinkedList<>();
          queue.add(createRootStep());
          return queue;
      }
docs: Returns the current thread's "initial value". Construct an new
  {@link java.util.Deque} with root step {@link #createRootStep()}
  
  @return the initial value for this thread-local