foo.cc: In function ‘bool permutation(int)’:
foo.cc:37:12: warning: control reaches end of non-void function [-Wreturn-type]
   37 | permutation(w-1);
      | ~~~~~~~~~~~^~~~~
foo.cc: In function ‘int main()’:
foo.cc:42:5: warning: iteration 24 invokes undefined behavior [-Waggressive-loop-optimizations]
   42 | k[i]=i;
      | ~~~~^~
foo.cc:41:16: note: within this loop
   41 | for(R int i=1;i<=25;i++)
      |               ~^~~~