<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def make_good_url(url=None, addition="/"):
 """Appends addition to url, ensuring the right number of slashes exist and the path doesn't get clobbered. 'http://www.server.com/anywhere/else' 'http://test.com/somewhere/over/the/rainbow/' 'None/' :param url: URL :param addition: Something to add to the URL :return: New URL with addition"""


    if url is None:
        return None

    if isinstance(url, str) and isinstance(addition, str):
        return "%s/%s" % (url.rstrip('/'), addition.lstrip('/'))
    else:
        return None