docs: 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