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())
      |                    ~~^~~~~~~~~~~~

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