def nextprefix(self):
        """
        Get the next available prefix.  This means a prefix starting with 'ns'
        with a number appended as (ns0, ns1, ..) that is not already defined
        on the wsdl document.
        """
        used = [ns[0] for ns in self.prefixes]
        used += [ns[0] for ns in self.wsdl.root.nsprefixes.items()]
        for n in range(0, 1024):
            p = 'ns%d' % n
            if p not in used:
                return p
        raise Exception('prefixes exhausted')