[fauigerzigerk]

>Double-entry, I suppose, means each event must be ingested exactly once by an even number of actors.)

No, the number of accounts (actors) does not have to be even. The sum of debits and credits has to be equal (or zero if you like).

[btown]

You're right - it was a silly thing for me to write! Something more accurate would be that because debits and credits must balance, there is no way to send a message that would only be seen by a single account (other than a no-op); thus, any meaningful transaction will have an impact on at least two accounts.

[fauigerzigerk]

Agreed.