foo.cc: In function 'std::string Add(std::string, std::string)':
foo.cc:37:22: 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:39:22: 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:70:23: 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:71:31: 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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