foo.cc: In function 'int main()':
foo.cc:15:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             while (j < primes.size()) {
                    ~~^~~~~~~~~~~~~~~
foo.cc:22:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 2; i < primes.size(); ++i) {
                     ~~^~~~~~~~~~~~~~~
foo.cc:28:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < small.size(); ++i)
                     ~~^~~~~~~~~~~~~~
foo.cc:31:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < big.size(); ++i)
                     ~~^~~~~~~~~~~~

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