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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 format_modified(self, modified, sep=" "):
 """Format modification date in UTC if it's not None. @param modified: modification date in UTC @ptype modified: datetime or None @return: formatted date or empty string @rtype: unicode """

        if modified is not None:
            return modified.strftime("%Y-%m-%d{0}%H:%M:%S.%fZ".format(sep))
        return u""