<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def normalize_url(base_url, rel_url):
 """ Normalize the `url` - from relative, create absolute URL. Args: base_url (str):
 Domain with ``protocol://`` string rel_url (str):
 Relative or absolute url. Returns: str/None: Normalized URL or None if `url` is blank. """

    if not rel_url:
        return None

    if not is_absolute_url(rel_url):
        rel_url = rel_url.replace("../", "/")

        if (not base_url.endswith("/")) and (not rel_url.startswith("/")):
            return base_url + "/" + rel_url.replace("../", "/")

        return base_url + rel_url.replace("../", "/")

    return rel_url