text: #ifndef NODE_H
  #define NODE_H
  
  
  class Node
  {   
  protected:
  
      enum class State{
          Error,
          Number
      };
  
      State m_state;
      double m_value;
  
  public:
      Node();
  
      double value() const;
      bool isValid() const;
  
      void setValue(double value);
  };
  
  #endif // NODE_H
  