def return_loan(actor, loan, audit):
    permitted = actor is not None and actor == loan["owner"] and loan["state"] == "open"
    audit.append({"action": "return", "outcome": "allowed" if permitted else "denied"})
    if not permitted:
        return False
    loan["state"] = "returned"
    return True

loan = {"owner": "borrower-18", "state": "open"}
audit = []
assert not return_loan(None, loan, audit)
assert not return_loan("borrower-19", loan, audit)
assert loan["state"] == "open"
assert return_loan("borrower-18", loan, audit)
assert not return_loan("borrower-18", loan, audit)
assert loan["state"] == "returned"
assert [event["outcome"] for event in audit] == ["denied", "denied", "allowed", "denied"]
assert all(set(event) == {"action", "outcome"} for event in audit)
print("anonymous denied; other denied; owner returned; repeat denied")