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2026-08-24 08:11:59 -07:00
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+# Ledger invariants
+
+`Ledger.apply(operation_id, account, delta)` applies an operation at most once globally.
+
+- A successful operation permanently consumes its operation ID.
+- A rejected overdraft must not consume its operation ID.
+- A caller may fund the account and retry the same rejected operation.
+- Balances may never become negative.
+- Duplicate successful operations return the current balance without applying the delta again.
+
+The current implementation violates the rejected-operation rule.