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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def image_placeholder(width: Union[int, str] = 1920, height: Union[int, str] = 1080) -> str: """Generate a link to the image placeholder. :param width: Width of image. :param height: Height of image. :return: URL to image placeholder. """

        url = 'http://placehold.it/{width}x{height}'
        return url.format(width=width, height=height)