foo.cc: In function 'int main()':
foo.cc:13:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   13 |     for (int i = 0; i < a.length(); i++) {
      |                     ~~^~~~~~~~~~~~

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