code: function rpoplpush(source, destination, req) {
    var src = this.getKey(source, req)
      , dest = this.getKey(destination, req)
      , element;
    if(src === undefined) return null;
    if(dest === undefined) {
      dest = new List();
      this.setKey(destination, dest, undefined, undefined, undefined, req);
    }
    element = src.rpop();
    if(src.llen() === 0) {
      this.delKey(source, req);
    }
    dest.lpush([element]);
    return element;
  }
docs: Atomically returns and removes the last element (tail) of the
  list stored at source, and pushes the element at the first
  element (head) of the list stored at destination.