<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def parse_size(size):
 """ Parses size string into a tuple :param size: String on the form '100', 'x100 or '100x200' :return: Tuple of two integers for width and height :rtype: tuple """

        if size.startswith('x'):
            return None, int(size.replace('x', ''))
        if 'x' in size:
            return int(size.split('x')[0]), int(size.split('x')[1])
        return int(size), None