foo.cc: In function 'bool just(int, int)':
foo.cc:8:29: warning: variable 'mid' set but not used [-Wunused-but-set-variable]
    8 |      double  a1=1,a2=1,a3=0,mid=0;
      |                             ^~~
foo.cc: In function 'int main()':
foo.cc:30:10: warning: unused variable 'a1' [-Wunused-variable]
   30 |      int a1=1,a2=1,a3=0,a,b;
      |          ^~
foo.cc:30:15: warning: unused variable 'a2' [-Wunused-variable]
   30 |      int a1=1,a2=1,a3=0,a,b;
      |               ^~
foo.cc:30:20: warning: unused variable 'a3' [-Wunused-variable]
   30 |      int a1=1,a2=1,a3=0,a,b;
      |                    ^~

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