/in/foo.cc: In function 'void insert(char*)':
/in/foo.cc:25:7: warning: array subscript has type 'char' [-Wchar-subscripts]
    s[k] = s[k - 1];
       ^
/in/foo.cc:26:6: warning: array subscript has type 'char' [-Wchar-subscripts]
   s[j] = ',';
      ^