code: func (x Pol) Expand() string {
  	if x == 0 {
  		return "0"
  	}
  
  	s := ""
  	for i := x.Deg(); i > 1; i-- {
  		if x&(1<<uint(i)) > 0 {
  			s += fmt.Sprintf("+x^%d", i)
  		}
  	}
  
  	if x&2 > 0 {
  		s += "+x"
  	}
  
  	if x&1 > 0 {
  		s += "+1"
  	}
  
  	return s[1:]
  }
docs: // Expand returns the string representation of the polynomial x.