def pin(self, hostname, x509):
        """
        Pin an :class:`OpenSSL.crypto.X509` object `x509` for use with the
        given `hostname`. Which information exactly is used to identify the
        certificate depends :meth:`_x509_key`.
        """

        key = self._x509_key(x509)
        self._storage.setdefault(hostname, set()).add(key)