Every movement on Holon is written this way, and the database enforces the rules rather than trusting the code. An entry whose lines do not sum to zero is refused when it commits. Rows can never be updated or deleted, so a mistake is corrected by a new entry that offsets it. Each entry carries a unique key, such as settle:<receipt>, so the same movement cannot be recorded twice.
A settlement and the receipt it pays for are written in the same transaction: there is no state where one exists without the other. Balances are sums of journal lines, which is why an audit can check the running gateway against the database line by line.
Example: settling a 0.035 EUR call on a forked agent writes four lines in one entry. The payer minus 0.035, Holon plus 0.0035, the fork's author plus 0.026775, the original author plus 0.004725.