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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 available(self):
 """ Returns a set of the available versions. :returns: A set of integers giving the available versions. """


        # Short-circuit
        if not self._schema:
            return set()

        # Build up the set of available versions
        avail = set(self._schema.__vers_downgraders__.keys())
        avail.add(self._schema.__version__)

        return avail