/in/foo.cc: In function 'int insert(const string&)':
/in/foo.cc:15:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < expression.length(); ++i) {
                     ~~^~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:18:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             while (i < expression.length() && expression[i] >= '0' && expression[i] <= '1') {
                    ~~^~~~~~~~~~~~~~~~~~~~~