/in/foo.cc: In function 'int main()':
/in/foo.cc:20:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i=0;i<b1.size();i++)
                      ~^~~~~~~~~~
/in/foo.cc:25:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i=0;i<b2.size();i++)
                      ~^~~~~~~~~~