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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 _relativeize(self, filename):
 """Return the portion of a filename that is 'relative' to the directories in this lookup. """


        filename = posixpath.normpath(filename)
        for dir in self.directories:
            if filename[0:len(dir)] == dir:
                return filename[len(dir):]
        else:
            return None