foo.cc: In function 'bool is(std::string)':
foo.cc:7:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    7 |  for (i = 0; i < s.size(); i++)
      |              ~~^~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:39:9: warning: unused variable 'i' [-Wunused-variable]
   39 |  int t, i, j, k;
      |         ^
foo.cc:39:12: warning: unused variable 'j' [-Wunused-variable]
   39 |  int t, i, j, k;
      |            ^
foo.cc:39:15: warning: unused variable 'k' [-Wunused-variable]
   39 |  int t, i, j, k;
      |               ^

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