/in/foo.cc: In function 'void output(std::vector<std::vector<int> >&)':
/in/foo.cc:6:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0;i < m.size();i++)
                 ~~^~~~~~~~~~
/in/foo.cc:8:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j = 0;j < m[i].size();j++)
                  ~~^~~~~~~~~~~~~