RPC(
  get_more,
  INT32_FIELD(msg_len)
  INT32_FIELD(request_id)
  INT32_FIELD(response_to)
  INT32_FIELD(opcode)
  INT32_FIELD(zero)
  CSTRING_FIELD(collection)
  INT32_FIELD(n_return)
  INT64_FIELD(cursor_id)
)