如何阻止延长舞台的动画片段边缘进入AS3中的舞台边界？

I have a background image that extends the stage, top and bottom, that's position is controlled by key presses which tweens the image. I want to stop the edge of the image at the top or bottom of the stage depending on which way the image is moving. I want this to apply to "layers_mc" and "bg_img".I have a background image that extends the stag