'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.'

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')
