foo.cc: In function 'int main()':
foo.cc:12:13: warning: unused variable 'in' [-Wunused-variable]
   12 |         int in = 0;
      |             ^~
foo.cc: In function 'std::vector<int> X(int, std::vector<int>)':
foo.cc:46:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   46 |         for (int i = 0; i < J.size(); i++)
      |                         ~~^~~~~~~~~~
foo.cc:50:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   50 |         for (int i = 0; i < J.size(); i++)
      |                         ~~^~~~~~~~~~
foo.cc:54:37: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   54 |                         if ((i + 1) < J.size())
      |                             ~~~~~~~~^~~~~~~~~~

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