code: func (x Pol) GCD(f Pol) Pol {
  	if f == 0 {
  		return x
  	}
  
  	if x == 0 {
  		return f
  	}
  
  	if x.Deg() < f.Deg() {
  		x, f = f, x
  	}
  
  	return f.GCD(x.Mod(f))
  }
docs: // GCD computes the Greatest Common Divisor x and f.