foo.cc: In member function 'BigInteger BigInteger::operator+(const BigInteger&) const':
foo.cc:50:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = dif; i < big.size(); ++i) res.push_back(big[i] + small[i - dif]);
                           ~~^~~~~~~~~~~~
foo.cc: In member function 'bool BigInteger::operator<=(const BigInteger&) const':
foo.cc:102:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int i = 0; i < num.size(); ++i)
                             ~~^~~~~~~~~~~~
foo.cc: In member function 'bool BigInteger::operator==(BigInteger&)':
foo.cc:127:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         else for (int i = 0; i < num.size(); ++i)
                              ~~^~~~~~~~~~~~

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