code: func opcodeToAltStack(op *parsedOpcode, vm *Engine) error {
  	so, err := vm.dstack.PopByteArray()
  	if err != nil {
  		return err
  	}
  	vm.astack.PushByteArray(so)
  
  	return nil
  }
docs: // opcodeToAltStack removes the top item from the main data stack and pushes it
  // onto the alternate data stack.
  //
  // Main data stack transformation: [... x1 x2 x3] -> [... x1 x2]
  // Alt data stack transformation:  [... y1 y2 y3] -> [... y1 y2 y3 x3]