/in/foo.cc: In function 'int Index(std::__cxx11::string, int, std::__cxx11::string)':
/in/foo.cc:8:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = k; i < s.size(); i++)
                     ~~^~~~~~~~~~
/in/foo.cc: In function 'void Output(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:18:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     while (id < s.size())
            ~~~^~~~~~~~~~