/in/foo.cc: In function 'int main()':
/in/foo.cc:27:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a.size(); i++)
                  ~~^~~~~~~~~~
/in/foo.cc:31:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < b.size(); i++)
                  ~~^~~~~~~~~~
/in/foo.cc:33:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j = 0; j < b[i].size(); j++)
                   ~~^~~~~~~~~~~~~