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Exercise 12. A return counts as timely

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# Exercise 12. A return counts as timely

An exercise for the [presumptions and
rebuttal](/tutorials/presumptions/) page. The archive issue journal is
kept by hand, and the archive rules establish: a handover record in the
journal means a timely return, unless the record itself implies
otherwise.

**Setup.** Write a presumption with three outcomes. A 20 March handover
record with a term to 31 March yields `TRUE_ONLY` on the timely-return
question. A 5 April record with the same term yields `FALSE_ONLY`: the
record itself proves the delay. A 20 March record disputed by the reader
yields `NEITHER`: the presumption is removed, but delay is not proven.

**Hint.** Rebuttal on the merits and blocking are different `unless`
clauses, and the first must bind every variable it reads.

Below is the solution. Try it yourself first.

## Solution

```law
language "law.core" version "0.2";
package tutorial.archive version "0.10.1";
namespace "urn:law:tutorial:archive";

entity Person;

relation document_lent(p: Person, issued: Date, due: Date) kind empirical { key(p); }
relation journal_return_entry(p: Person, on: Date) kind empirical { key(p); }
relation journal_entry_disputed(p: Person) kind empirical;
relation returned_in_time(p: Person) kind institutional;
```

The presumption. The first `unless` rebuts on the merits and says what
is established instead; the second only blocks:

```law
presumption TimelyReturnByJournal {
    for p: Person;
    for issued, due, on: Date;
    when document_lent(p, issued, due) and journal_return_entry(p, on);
    presume returned_in_time(p);
    unless document_lent(p, issued, due) and journal_return_entry(p, on) and on > due
        then not returned_in_time(p);
    unless journal_entry_disputed(p);
}
```

The `document_lent` and `journal_return_entry` repetition in the first
`unless` is no accident: the clause reads `due` and `on`, and the
compiler requires it to bind them itself. Remove the repetition and you
get `LDC-E4101`.

## Check

| Facts | Answer on `returned_in_time(ivanova)` |
|---|---|
| term to 31 March, handover record 20 March | `TRUE_ONLY` |
| term to 31 March, handover record 5 April | `FALSE_ONLY` |
| term to 31 March, 20 March record, record disputed | `NEITHER` |
| term exists, no handover record | `NEITHER` |

What the exercise teaches. A presumption's three outcomes are three
different claims about the world: "counts", "the opposite proven",
"does not count, but not refuted either". A disputed record does not make
the return untimely; it merely strips the reader of the presumption, and
the question is then settled by evidence.