#include <iostream>
#include <string>
#include <functional>
 
using namespace std;
 
void x(ostream &os) {
  os << "x()" << std::endl;
}
 
template <class T>
void y(void (*f)(ostream &), T &os) {
  std::cout << "y(): ";
  (*f)(os);
}
 
template <class T>
void z(void (*f)(ostream &), T os) {
  std::cout << "z(): ";
  (*f)(os);
}
 
int main() {
  x(cout);
  y(x, cout);
  //z(x, cout); /* No Good !! because "basic_ostream(const basic_ostream&) = delete;" */
  z(x, ref(cout)); /* OK. */
 return 0;
}
/* end */
 

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