foo.cc: In function 'int main()':
foo.cc:12:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 1; i <= s1.size(); i++)
                         ~~^~~~~~~~~~~~
foo.cc:14:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int j = 1; j <= s2.size(); j++)
                         ~~^~~~~~~~~~~~
foo.cc:16:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 1; i <= s1.size(); i++)
                         ~~^~~~~~~~~~~~
foo.cc:18:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 1; j <= s2.size(); j++)
                             ~~^~~~~~~~~~~~

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