foo.cc: In function 'bool solve(std::string&)':
foo.cc:79:44: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   79 | #define _for(i, a, b) for ( int i = (a); i < (b); ++i)
      |                                            ^
foo.cc:95:9: note: in expansion of macro '_for'
   95 |         _for(i,0,str.size())
      |         ^~~~

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