foo.cc: In function 'int Val(std::string)':
foo.cc:8:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    8 |  for (int i = 0; i < s.length(); i++)
      |                  ~~^~~~~~~~~~~~
foo.cc:14:8: warning: unused variable 'opd2' [-Wunused-variable]
   14 |    int opd2 = OPD.top(); OPD.pop();
      |        ^~~~
foo.cc:15:8: warning: unused variable 'opd1' [-Wunused-variable]
   15 |    int opd1 = OPD.top(); OPD.pop();
      |        ^~~~

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