RPC(
  reply,
  INT32_FIELD(msg_len)
  INT32_FIELD(request_id)
  INT32_FIELD(response_to)
  INT32_FIELD(opcode)
  ENUM_FIELD(flags)
  INT64_FIELD(cursor_id)
  INT32_FIELD(start_from)
  INT32_FIELD(n_returned)
  BSON_ARRAY_FIELD(documents)
)