foo.cc: In function 'int main()':
foo.cc:26:25: warning: array subscript has type 'char' [-Wchar-subscripts]
   26 |         A += a[ac[i % na]][bc[i % nb]];
      |                ~~~~~~~~~^
foo.cc:26:37: warning: array subscript has type 'char' [-Wchar-subscripts]
   26 |         A += a[ac[i % na]][bc[i % nb]];
      |                            ~~~~~~~~~^
foo.cc:27:25: warning: array subscript has type 'char' [-Wchar-subscripts]
   27 |         B += a[bc[i % nb]][ac[i % na]];
      |                ~~~~~~~~~^
foo.cc:27:37: warning: array subscript has type 'char' [-Wchar-subscripts]
   27 |         B += a[bc[i % nb]][ac[i % na]];
      |                            ~~~~~~~~~^

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