foo.cc: In function 'v mul(v, v)':
foo.cc:26:18: 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<a.size();i++)
      |                 ~^~~~~~~~~
foo.cc:27:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   27 |         for(int j=0;j<b.size();j++)
      |                     ~^~~~~~~~~

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