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.

OverviewBoundariesCorpus formsPitfalls

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).

Reserved and closed

• prohibition as sugar is retained (§130: 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.
• presumption vs defeasible rule + unless: a repeatable proof scheme with named nodes vs a one-off exception. See the defeasible-rules page.
• 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.