foo.cc: In function 'int main()':
foo.cc:7:23: error: expected primary-expression before ':' token
    7 |                 case +:cout<<a+c<<endl;
      |                       ^
foo.cc:8:23: error: expected primary-expression before ':' token
    8 |                 case -:cout<<a-c<<endl;
      |                       ^

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