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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 vert_dpi(self):
 """ Integer dots per inch for the height of this image. Defaults to 72 when not present in the file, as is often the case. """

        pHYs = self._chunks.pHYs
        if pHYs is None:
            return 72
        return self._dpi(pHYs.units_specifier, pHYs.vert_px_per_unit)