'Store a transform with an associated `pending` node.'

def add_pending(self, pending, priority=None):

    transform_class = pending.transform
    if (priority is None):
        priority = transform_class.default_priority
    priority_string = self.get_priority_string(priority)
    self.transforms.append((priority_string, transform_class, pending, {}))
    self.sorted = 0
