foo.cc: In function 'int main()':
foo.cc:18:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   18 | for (k = 0; k <a.size(); k++)
      |             ~~^~~~~~~~~
foo.cc:61:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   61 | for (k = 0; k < a.size(); k++)
      |             ~~^~~~~~~~~~

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