/in/foo.cc: In function 'std::__cxx11::string mup(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:8:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.size();i++){
              ~^~~~~~~~~
/in/foo.cc:9:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=0;j<b.size();j++){
               ~^~~~~~~~~