foo.cc: In function 'int fen(std::string&, char (*)[10001])':
foo.cc:10:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   10 |  for(int i=0;i<a.length();i++)
      |              ~^~~~~~~~~~~
foo.cc:22:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   22 |  for(int i=0;i<a.length();i++)
      |              ~^~~~~~~~~~~

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