/in/foo.cc: In function 'int main()':
/in/foo.cc:37:56: warning: variable 'ca' set but not used [-Wunused-but-set-variable]
  Num ab=f(a,b),bc=f(b,c),ac=f(a,c),cb=f(c,b),ba=f(b,a),ca=f(c,a);
                                                        ^~