@classmethod
def genprefix(cls, node, ns):
    """
        Generate a prefix.
        @param node: An XML node on which the prefix will be used.
        @type node: L{sax.element.Element}
        @param ns: A namespace needing an unique prefix.
        @type ns: (prefix, uri)
        @return: The I{ns} with a new prefix.
        """
    for n in range(1, 1024):
        p = 'ns%d' % n
        u = node.resolvePrefix(p, default=None)
        if u is None or u == ns[1]:
            return p, ns[1]
    raise Exception('auto prefix, exhausted')
