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
Return the portion of a filename that is 'relative'
           to the directories in this lookup.