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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 to_bing_str(self):
 """ Convert Viewbox object to a string that can be used by Bing as a query parameter. """

        vb = self.convert_srs(4326)
        return '%s,%s,%s,%s' % (vb.bottom, vb.left, vb.top, vb.right)