text: #include <string>
  
  extern std::string keyA;
  extern std::string keyAB;
  extern std::string keyAC;
  extern std::string keyAD;
  extern std::string keyB;
  extern std::string keyBC;
  extern std::string keyBD;
  extern std::string keyC;
  extern std::string keyCD;
  extern std::string keyD;
  