code: func (s *stack) Tuck() error {
  	so2, err := s.PopByteArray()
  	if err != nil {
  		return err
  	}
  	so1, err := s.PopByteArray()
  	if err != nil {
  		return err
  	}
  	s.PushByteArray(so2) // stack [... x2]
  	s.PushByteArray(so1) // stack [... x2 x1]
  	s.PushByteArray(so2) // stack [... x2 x1 x2]
  
  	return nil
  }
docs: // Tuck copies the item at the top of the stack and inserts it before the 2nd
  // to top item.
  //
  // Stack transformation: [... x1 x2] -> [... x2 x1 x2]