foo.cc: In function 'std::string Add(std::string&, std::string&)':
foo.cc:37:8: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   37 |   if(i < x.length())
      |      ~~^~~~~~~~~~~~
foo.cc:39:8: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   39 |   if(i < y.length())
      |      ~~^~~~~~~~~~~~
foo.cc: In function 'std::string Mul(std::string&, std::string&)':
foo.cc:70:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   70 |  for(int i=0; i<x.length(); i++)
      |               ~^~~~~~~~~~~
foo.cc:71:17: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   71 |   for(int j=0; j<y.length(); j++)
      |                ~^~~~~~~~~~~

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