/in/foo.cc: In function 'std::__cxx11::string sub(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:55:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a.size(); i++)
                  ~~^~~~~~~~~~
/in/foo.cc: In function 'std::__cxx11::string pow_mod(std::__cxx11::string)':
/in/foo.cc:141:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^