code: @Nullable
      @Override
      public Integer getSuccessor(Integer state, @Nullable I input) {
          final Integer result;
          if (state < word.length() && input.equals(word.getSymbol(state))) {
              result = state + 1;
          } else {
              result = null;
          }
  
          return result;
      }
docs: Get the successor state of a given state, or {@code null} when no such successor exists.
  
  @see SimpleDTS#getSuccessor(Object, Object)