foo.cc: In function 'bool is_p(int)':
foo.cc:3:32: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    3 | #define fr(i,a,b) for(int i=a;i<b;i++)
......
    7 |  fr(i,0,s.size()/2)
      |     ~~~~~~~~~~~~~~              
foo.cc:7:2: note: in expansion of macro 'fr'
    7 |  fr(i,0,s.size()/2)
      |  ^~

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