code: def genprefix(cls, node, ns):
          
          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')
docs: 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.