foo.cc: In function 'std::vector<int> operator*(std::vector<int>, std::vector<int>)':
foo.cc:42:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < a.size(); i++) {
                     ~~^~~~~~~~~~
foo.cc:46:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int j = 0; j < b.size(); j++) {
                         ~~^~~~~~~~~~

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