Proposal: Core structural predicates for envelope-to-envelope relationships

[ChristopherA]

During work on the attribution predicates BCR suite, I identified a category of predicates that are fundamentally different from semantic predicates: structural predicates whose range is Digest only.

These predicates express cryptographically verifiable relationships between envelopes. They're envelope infrastructure, not semantic claims.

The Key Insight

What makes a predicate "structural" in the envelope context is that its range is Digest only. This means:

1. The relationship is content-addressed (cryptographically verifiable)
2. The relationship works whether the target is revealed OR elided
3. The predicate expresses envelope-to-envelope topology, not semantic meaning

This is distinct from predicates in existing vocabularies (Dublin Core, PROV-O, etc.) which operate on URIs. URI-based relationships require resolution; digest-based relationships are self-verifying.

Proposed Predicates (5 total)

supersedes — Version replacement

// With content: "v2 replaces v1"
{ Digest(policy-v2) ['supersedes': Digest(policy-v1)] }

// With ELIDED: "v2 replaces something (verify if you have v1)"
{ Digest(policy-v2) ['supersedes': ELIDED] }

derivedFrom — Lineage (different thing from source)

// With content: "This analysis came from that dataset"
{ Digest(quarterly-analysis) ['derivedFrom': Digest(sales-data-q4)] }

// With ELIDED: "This analysis came from a private source"
{ Digest(quarterly-analysis) ['derivedFrom': ELIDED] }

includes — Composition

// With content: "This collection contains these items"
{ Digest(audit-bundle) ['includes': Digest(finding-1), 'includes': Digest(finding-2)] }

// With ELIDED: "This collection contains something (membership proof)"
{ Digest(audit-bundle) ['includes': ELIDED] }

reveals — Elision relationship

// The revealed envelope proves it matches the elided form
{ Digest(full-document) ['reveals': Digest(redacted-document)] }

Unique: Works even when digests are salted differently — it's about the cryptographic operation.

about — Subject relationship

// With content: "This review is about these things"
{
    Digest(code-review) [
        'about': Digest(pull-request-123)
        'about': Digest(security-policy)
    ]
}

// With ELIDED: "This review is about something (verify if you have it)"
{
    Digest(code-review) [
        'about': ELIDED
        'reviewer': XID(alice)
    ]
}

Why about when envelopes have subjects?

Aspect	Subject position	about predicate
Cardinality	Exactly one	Zero or more
Meaning	Identity (what this IS)	Reference (what this RELATES TO)
When ELIDED	Envelope loses identity	Envelope keeps identity

The ELIDED case is the key distinction:

// Subject elided — you don't know WHAT this is
{ ELIDED ['reviewer': XID(alice)] }

// Subject present, about elided — you know it's a review, not what it reviewed
{ Digest(code-review) ['about': ELIDED, 'reviewer': XID(alice)] }

The second pattern lets you prove "Alice reviewed something" without revealing what. Subject elision alone can't express this.

Why These Five?

supersedes — Version chains. PROV-O's wasRevisionOf validates this as distinct from derivation. "Same thing, newer version."

derivedFrom — Lineage. PROV-O's wasDerivedFrom is the established pattern. "Different thing, from source." Distinct from supersedes.

includes — Composition. Universal across Git (tree→blob), LDP (ldp:contains), Dublin Core (hasPart). Merkle trees are built on this.

reveals — Unique to content-addressed systems with elision. Expresses the cryptographic operation of revealing elided content. Works even when digests are salted differently.

about — Subject relationship. The structural primitive for "this envelope is about that thing." Web Annotation's oa:hasTarget is the closest analog.

Research Validation

I reviewed structural relationships across:

• Content-addressed systems (IPLD, Git, CAS)
• Provenance ontologies (PROV-O)
• Metadata standards (Dublin Core)
• Semantic web (RDF/OWL, SKOS)
• Linked data (LDP)
• Web Annotation

The research confirmed these five cover the structural primitives. Other candidates (quotedFrom, requires, sameAs, references) are either semantic specializations or redundant with hash equality.

Why Core (<1000)?

1. Range is Digest only — These operate on content-addressed identifiers, not URIs or text
2. Cryptographically verifiable — Unlike URI-based relationships, these can be verified by hashing
3. Envelope infrastructure — They express how envelopes relate to each other structurally
4. Enable privacy patterns — Combined with ELIDED, they enable privacy-preserving cryptographic commitments

Example: Privacy-Preserving Attestation

{
    Digest(security-audit) [
        'about': ELIDED
        'processDisclosure': "Conducted under NDA"
    ]
}

The verifier knows:

• An audit exists with a specific digest
• It's about something (can verify if they have the subject)
• It was conducted under NDA

Without revealing what was audited.

Open Question: attestsTo

Is attestsTo a sixth structural predicate, or is it about with semantic context?

// Option A: attestsTo is structural (6th predicate)
{ Digest(endorsement) ['attestsTo': ELIDED] }

// Option B: about + semantic wrapper
{ Digest(endorsement) ['about': ELIDED, 'isA': 'Attestation'] }

The ELIDED pattern suggests attestsTo might be structural — "this exists to attest to that" is a content-agnostic relationship. But it may be sufficiently covered by about.

Research Questions for Wolf

1. reveals mechanics: Does reveals work across different salting? My understanding is it's about the cryptographic operation, not just digest equality.

2. attestsTo placement: Is attestsTo structurally different from about, or is attestation a semantic layer on top of about?

3. Existing coverage: Are any of these already covered by core predicates? Current registry (0-706) doesn't appear to have digest-only predicates.

4. Codepoint range: If these belong in core, what range? Following XID privileges (ends at 86), perhaps 87-92?

Context

This emerged from reviewing supersedes (currently proposed at 1000 in BCR-2026-005). The realization that its range is Digest suggests it may belong in core with other structural predicates.

Related: BCR-2026-005 General Assertion Predicates (PR #150)

/cc @shannona