foo.cc: In function 'std::ostream& operator<<(std::ostream&, string3x)':
foo.cc:6:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < v.size(); i++) {
                 ~~^~~~~~~~~~
foo.cc:7:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j = 0; j < v[i].size(); j++)
                  ~~^~~~~~~~~~~~~

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