def secret(self, s):
        """
        Parse text either a private key or a private hierarchical key.
        Return a subclass of :class:`Key <pycoin.key.Key>`, or None.
        """
        s = parseable_str(s)
        for f in [self.private_key, self.hierarchical_key]:
            v = f(s)
            if v:
                return v