foo.cc: In function 'int main()':
foo.cc:26:24: error: 'mod' was not declared in this scope
  cout<<a<<"^"<<b<<" "<<mod<<" "<<c<<"="<<cy(a,b,c)<<endl;
                        ^~~
foo.cc:26:24: note: suggested alternative: 'void'
  cout<<a<<"^"<<b<<" "<<mod<<" "<<c<<"="<<cy(a,b,c)<<endl;
                        ^~~
                        void

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