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Presumptions and fictions: boundaries

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# Presumptions and fictions: boundaries

> Updated 3 October 2026: `concept`, `except_when`, `claim`, `definition … sufficient`, and `default` strength have been removed, together with their special deprecation diagnostic. References below to the former behavior and corpus sources are historical. New source uses `relation`, `unless`, `duty`, a strict rule, and `defeasible`.

What the construct does NOT do in 0.2: limits, closed lists,
neighbouring constructs, and the selection rule.

## Does not do

- **Does not pick strength for the author.** On a fiction, strength after
  the name is mandatory (`strict`/`defeasible`); the core does not pick a
  default. `defeater` as a classification strength is inadmissible: a
  class is either supported, or the exception is a separate rule.
- **Does not apply contraposition.** A definition does not convert:
  the reverse direction is only a separate rule with a literal head. `exact`
  does not create contraposition; an explicit negative classification is a
  separate rule.
- **Does not expand foreign clauses.** `scope`/`effective`/source anchors/
  metadata in presumption, fiction, classification, and definition bodies in
  0.1 are `LDC-E0201`. Their expansion is a separate
  subject; until then the rejection is loud.
- **Does not grant victory without an edge.** A later chain link without the
  chain edge loses victory over the general rule: Contradiction instead of
  Refuted.
- **Does not create predicates from typos.** A definition synthesizes its
  own relation with the declared signature; publish the definition itself
  when the vocabulary must be public ([§144](https://github.com/arxohq/law/blob/master/spec/SPEC.ru/20-part-xix-definitions-presumptions-fictions-exceptions.ru.md#144-definition)).

## Reserved and closed

- `prohibition` as sugar is retained ([§130](https://github.com/arxohq/law/blob/master/spec/SPEC.ru/18-part-xvii-normative-positions.ru.md#130-prohibition-sugar): the act vocabulary matches the form).
- Generated-node names are fixed: the presume-rule is
  `<presumption>/R1`, exceptions in order `/R2`, `/R3`, …; a named `unless
  Exempt` is `<base>/unless/Exempt`. Renaming changes the
  identifier and the hash; reordering exceptions may change the chain
  meaning.
- A legal definition must have scope (`for_purposes_of`, `effective`): one
  term means different definitions in different packages.

## Neighbours and selection rule

- `presumption` vs `definition`: “counts as, if, until rebutted” vs
  “is”. A burden of rebuttal needed — a presumption. See the
  [definitions page](/constructs/definition/).
- `presumption` vs `defeasible rule + unless`: a repeatable proof scheme
  with named nodes vs a one-off exception. See the
  [defeasible-rules page](/constructs/rule-defeasible-unless/).
- `presumption` vs `classification defeasible`: an exception with priority
  vs class support without a defeater.
- `fiction deem` vs `rule then`: institutional equation vs a fact
  about the world. A fiction head is institutional.
- `unless C` vs `unless C then not A`: a defeater with Not established, not refuted vs
  a contrary rule with Refuted and priority.
- `definition necessary` vs `constraint`: a concept half with
  `was_derived_from` vs a one-off check. See the
  [constraints page](/constructs/constraint/).