foo.cc: In function 'void Output(std::vector<std::vector<char> >&)':
foo.cc:13:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<char> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   13 |         for(int i=0; i<m.size(); i++)
      |                      ~^~~~~~~~~
foo.cc:15:31: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   15 |                 for(int j=0; j<m[i].size(); j++)
      |                              ~^~~~~~~~~~~~

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