https://gitlab.synchro.net/main/sbbs/-/merge_requests/474#note_6035

This is not the proper criteria to determine a message is a ballot.

Compare the message header's `type` property, if its value is equal to `MSG_TYPE_BALLOT`, it's a ballot.

If you want a faster method (no need to read message header), you can tell a message is a ballot from the message's index record: bit-wise AND the index `attr` property with `MSG_VOTE`, if the result is non-zero, message is a ballot:
```
if ((index.attr & MSG_VOTE) != 0) { /* message is a ballot */ }
```
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