'Pass a call name as the first argument and a generator
is returned for the initial response and any continuation
call responses made using the NextToken.'

def iter_call(self, call, *args, **kw):

    method = self.method_for(call)
    assert method, 'No    call    named    "{0}"'.format(call)
    return self.iter_response(method(*args, **kw))
