code: def nextprefix(self):
          
          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')
docs: 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.