foo.cc: In function 'int main()':
foo.cc:34:35: error: 'xi' was not declared in this scope; did you mean 'i'?
   34 |                                 v[xi][yi]=false;
      |                                   ^~
      |                                   i
foo.cc:34:39: error: 'yi' was not declared in this scope; did you mean 'i'?
   34 |                                 v[xi][yi]=false;
      |                                       ^~
      |                                       i
foo.cc:29:14: warning: unused variable 'ac' [-Wunused-variable]
   29 |         char ac;
      |              ^~

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