ADR-126: A reference may name several schemes

Context

A typed reference named exactly one scheme (ADR-060), so a relation whose legitimate targets span two families could not be declared. #160 found the case in examples/constitution: a boundary that overrides another boundary is rejected as “not a PRACTICE code”, though a carve-out that beats a prohibition is the same relation whichever kind of rule it beats. The same shape appears in an anthology where a practice cites a paper or a dataset, and in a spec that supersedes a spec or an RFC.

It came up again from a downstream record. Nucleation, which keeps literature (LIT) and accounts drawn from it (THEORY), is designing schemes for developing its owner’s own arguments. Its candidate CLAIM scheme needs premises that are THEORY entries, other CLAIMs or worked cases. Under one scheme per reference, that is three parallel fields per relation.

Decision

scheme: takes a string or a list of strings:

rests_on:
  scheme: [LIT, CASE]
  many: true
  converse: supports

• Membership, not equality. A code passes if it belongs to any scheme listed, and resolves in its own scheme. The finding names the union: “is not a LIT or CASE code”. Every check that compared a code’s scheme against the declared one now tests membership instead: the contract, the typed edges, luria relate, ref:: statements, grouped views and invariants.
• A string is a list of one. Internally Reference.scheme and Field.reference are tuples, so there is one representation and no branch on the shape.
• One relation, one name. A converse lives on every scheme listed, by the same field name, and each must name the declaring scheme among its own targets (ADR-097 applied per target). relations.pairs() yields one pair per target scheme. Each target holds its own half, so the fixer completes each one independently, and the existing machinery for a crossing pair needed no other change.
• A chain still walks within one scheme. A chain may walk a relation only if the chain’s own scheme is its sole target. A union that includes the chain’s scheme can still leave it, and a chain is a sequence within one scheme (ADR-106). To assert a shared field over the relation, declare invariant on it. That is now checked against every target scheme.
• A followed derivation is checked against every target. from: reads whichever scheme the code names, so a template that only some targets can render is refused.

Alternatives considered

• A field per target family (overrides_practice, overrides_boundary). This is what a project had to do before this change. It moves the type system’s job into the field name, reads as two relations, and leaves every reader of the graph to know which fields to union.
• A loose requires field. This checks presence and nothing else, which is the failure ADR-060 was written against.
• A refinement predicate (Ref[LIT where status = Active]). #141 defers this. A union of schemes is strictly less expressive, needs none of the predicate machinery, and widens an existing check rather than adding a new kind of check.
• Letting a chain walk a union that includes its own scheme, skipping the edges that leave it. A chain would then silently drop edges it was declared over, and that is how a rendered line comes to omit a step without a finding.
• A different converse name per target scheme. This would let LIT call the relation supports and CASE call it grounds. A converse is the same relation read backwards, and a second name would let two readings of one fact disagree about what they are called.

Consequences

• Existing configs are unchanged: every string target becomes a one-element tuple, and the full test suite passes. Seven assertions that compared the internal representation to a bare string were updated to the tuple.
• Code that reads Reference.scheme or Field.reference must treat them as tuples. contract.targets, contract.target_of and contract.spelled are the helpers, and nothing should compare a code’s prefix against the declaration by hand.
• relations.converse_scheme_of is removed. It had no callers, and for a multi-scheme field it would have named one target out of several.
• examples/constitution declares overrides: [PRACTICE, BOUNDARY], the shape #160 asked for. It was fired on the example before being trusted (see the devlog of 2026-10-07).