foo.cc: In function 'void Mul(std::vector<int>&, std::vector<int>&, std::vector<int>&)':
foo.cc:22:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   22 |         for(int i=0; i<a1.size(); i++)
      |                      ~^~~~~~~~~~
foo.cc:23:31: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   23 |                 for(int j=0; j<a2.size(); j++)
      |                              ~^~~~~~~~~~
foo.cc:26:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   26 |         for(int i=0; i<a3.size(); i++)
      |                      ~^~~~~~~~~~

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