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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 _prefix_from_prefix_string(self, prefixlen_str):
 """Turn a prefix length string into an integer. Args: prefixlen_str: A decimal string containing the prefix length. Returns: The prefix length as an integer. Raises: NetmaskValueError: If the input is malformed or out of range. """

        try:
            if not _BaseV4._DECIMAL_DIGITS.issuperset(prefixlen_str):
                raise ValueError
            prefixlen = int(prefixlen_str)
            if not (0 <= prefixlen <= self._max_prefixlen):
               raise ValueError
        except ValueError:
            raise NetmaskValueError('%s is not a valid prefix length' %
                                    prefixlen_str)
        return prefixlen