amduat/tier1/pel-program-dag-desc-1.md

# PEL/PROGRAM-DAG-DESC/1 — Scheme Descriptor for DAG Program Scheme

Status: Approved
Owner: Niklas Rydberg
Version: 0.1.6
SoT: Yes
Last Updated: 2025-11-16
Linked Phase Pack: N/A
Tags: [composition, registry]

<!-- Source: /amduat/docs/new/pel-program-dag-desc.md | Canonical: /amduat/tier1/pel-program-dag-desc-1.md -->

**Document ID:** `PEL/PROGRAM-DAG-DESC/1`
**Layer:** L1 Scheme Descriptor (binds PEL scheme to ASL values)

**Depends on (normative):**

* `ASL/1-CORE v0.4.x` — Artifact substrate (`Artifact`, `TypeTag`, `Reference`, `EncodingProfileId`)
* `ENC/ASL1-CORE v1.0.x` — canonical encoding profile (`ASL_ENC_CORE_V1`)
* `PEL/1-CORE v0.3.x` — primitive execution layer (schemes and `Exec_s`)
* `PEL/PROGRAM-DAG/1 v0.3.x` — DAG program scheme (`Program`, `Node`, `Exec_DAG`, symbolic `SchemeRef_DAG_1`)
* `ENC/PEL-PROGRAM-DAG/1 v0.2.x` — canonical encoding for DAG programs (Program Artifacts, profile ID `PEL_ENC_PROGRAM_DAG_V1`)

**References (informative):**

* `HASH/ASL1 v0.2.x` — ASL1 hash family (`HashId` assignments, `HASH-ASL1-256`)
* `SUBSTRATE/STACK-OVERVIEW v0.3.x` — stack layering and kernel invariants
* `ENC/PEL-TRACE-DAG/1 v0.1.x` — canonical encoding for DAG traces (Trace Artifacts)
* `OPREG/PEL1-KERNEL` and related param profiles — operation registry for DAG ops (context only)

© 2025 Niklas Rydberg.

## License

Except where otherwise noted, this document (text and diagrams) is licensed under
the Creative Commons Attribution 4.0 International License (CC BY 4.0).

The identifier registries and mapping tables (e.g. TypeTag IDs, HashId
assignments, EdgeTypeId tables) are additionally made available under CC0 1.0
Universal (CC0) to enable unrestricted reuse in implementations and derivative
specifications.

Code examples in this document are provided under the Apache License 2.0 unless
explicitly stated otherwise. Test vectors, where present, are dedicated to the
public domain under CC0 1.0.

---

## 0. Overview

`PEL/PROGRAM-DAG/1` defines the **DAG program scheme** for PEL/1-CORE and refers to its scheme identifier symbolically as `SchemeRef_DAG_1`.

`PEL/PROGRAM-DAG-DESC/1` defines:

* the **logical descriptor value** for the DAG scheme,
* its realization as a concrete **ASL/1 Artifact** (the *descriptor Artifact*), and
* how engines derive and recognize `SchemeRef_DAG_1` as the scheme’s `SchemeRef`.

In particular, this document:

* binds the DAG scheme semantics (`Exec_DAG`) to a specific `SchemeRef` value;
* specifies the descriptor’s **TypeTag** and internal structure;
* specifies how `SchemeRef_DAG_1` is derived from the descriptor Artifact under `ASL_ENC_CORE_V1` and a chosen hash algorithm; and
* fixes a **canonical** `SchemeRef_DAG_1` for the Amduat 2.0 baseline profile using `HASH-ASL1-256` (`HashId = 0x0001`),

without introducing dependencies on stores, TGK, certificates, facts, or overlays.

---

## 1. Scope & Layering

### 1.1 Purpose

This specification defines:

* The **logical structure** of the DAG scheme descriptor value (`DagSchemeDescriptor`).

* The **descriptor Artifact** that carries this value:

  * its `type_tag` (a “PEL scheme descriptor” `TypeTag`), and
  * its canonical `bytes` layout.

* The **derivation** of `SchemeRef_DAG_1` as a `Reference` to that descriptor Artifact.

* The **binding** between `SchemeRef_DAG_1` and the DAG scheme defined in `PEL/PROGRAM-DAG/1`.

It does **not** redefine:

* DAG program semantics or structure (`Program`, `Node`, `Exec_DAG`),
* Program or trace encodings (`ENC/PEL-PROGRAM-DAG/1`, `ENC/PEL-TRACE-DAG/1`),
* hash algorithms or `HashId` (`HASH/ASL1`),
* store, graph, certification, or provenance behavior.

### 1.2 Layering constraints

In line with `SUBSTRATE/STACK-OVERVIEW`:

* `PEL/PROGRAM-DAG-DESC/1` is a **scheme descriptor**:

  * it binds a PEL scheme (`PEL/PROGRAM-DAG/1`) to a specific ASL/1 `Reference`; and
  * it describes how engines recognize and interpret that scheme.

* It MUST NOT introduce dependencies on:

  * stores (`ASL/1-STORE`),
  * graph kernels or graph stores (`TGK/1-CORE`, `TGK/STORE/1`),
  * certification, evidence, facts, or overlays (`CIL/1`, `FER/1`, `FCT/1`, `OI/1`).

* It MAY refer informatively to:

  * canonical trace encodings (`ENC/PEL-TRACE-DAG/1`),
  * operation registries and parameter profiles (`OPREG/PEL1-KERNEL`, etc.),

but MUST NOT depend on them for its core semantics.

---

## 2. Dependencies, Terminology & Conventions

RFC 2119 terms (**MUST**, **SHOULD**, **MAY**, etc.) are normative.

### 2.1 From ASL/1-CORE

```text
Artifact {
  bytes:    OctetString
  type_tag: optional TypeTag
}

TypeTag {
  tag_id: uint32
}

Reference {
  hash_id: HashId
  digest:  OctetString
}

HashId            = uint16
EncodingProfileId = uint16
```

This spec treats:

* `Artifact` as the unit of identity and content,
* `TypeTag` as the typing hint for `Artifact.bytes`,
* `Reference` as the content address for Artifacts, and
* `EncodingProfileId` as the identifier of encoding profiles (e.g. `ASL_ENC_CORE_V1`, `PEL_ENC_PROGRAM_DAG_V1`).

### 2.2 From ENC/ASL1-CORE v1

`ENC/ASL1-CORE v1` defines:

* `ArtifactBytes = encode_artifact_core_v1(Artifact)` — canonical encoding for Artifacts.
* `ReferenceBytes = encode_reference_core_v1(Reference)` — canonical encoding for References.

It also instantiates the generic ASL rule:

```text
ArtifactBytes = encode_P(A)
digest        = H(ArtifactBytes)
Reference     = { hash_id = HID, digest = digest }
```

for `P = ASL_ENC_CORE_V1` and any hash algorithm `H` with `HashId = HID`.

### 2.3 From PEL/1-CORE and PEL/PROGRAM-DAG/1

From `PEL/1-CORE`:

```text
SchemeRef = Reference

Exec_s :
  (program: Artifact, inputs: list<Artifact>, params: optional Artifact)
    -> (outputs: list<Artifact>, result: ExecutionResultValue)
```

From `PEL/PROGRAM-DAG/1`:

* The DAG scheme is a concrete scheme over PEL/1-CORE:

  ```text
  Exec_DAG :
    (program: Artifact, inputs: list<Artifact>, params: optional Artifact)
      -> (outputs: list<Artifact>, result: ExecutionResultValue)
  ```

* This scheme is identified symbolically as `SchemeRef_DAG_1`, with:

  ```text
  SchemeRef = SchemeRef_DAG_1
  Exec_s    = Exec_DAG
  ```

`PEL/PROGRAM-DAG-DESC/1` makes this symbolic `SchemeRef_DAG_1` concrete.

### 2.4 Conventions

Unless stated otherwise:

* Integers:

  * `u8`  — 1 byte, unsigned.
  * `u16` — 2 bytes, unsigned, big-endian.
  * `u32` — 4 bytes, unsigned, big-endian.

* `Utf8String` is encoded as:

  ```text
  Utf8String =
    length (u32)
    bytes[0..length-1]
  ```

  where:

  * `length` is the number of bytes,
  * `bytes` MUST be well-formed UTF-8,
  * there is no terminator or padding.

* Embedded `Reference` values use a length-prefixed wrapper over canonical `ReferenceBytes` from `ENC/ASL1-CORE v1`:

  ```text
  EncodedRef =
    ref_len   (u32)
    ref_bytes (byte[0..ref_len-1])  // canonical ReferenceBytes
  ```

  where:

  * `ref_bytes` MUST be the canonical `ReferenceBytes` for some `Reference` value under `ENC/ASL1-CORE v1`,
  * `ref_len` MUST be the exact length of `ref_bytes` (MUST be ≥ 2).

  Decoders MUST:

  * treat `ref_len < 2` as an encoding error, and
  * decode `ref_bytes` using `decode_reference_core_v1`. Any failure in that decoding (including, when `HASH/ASL1` is implemented, a digest-length mismatch for a known `hash_id`) MUST be treated as an encoding error for `DagSchemeDescriptorBytes`.

* Optional `Reference` fields use:

  ```text
  OptionalEncodedRef =
    has_ref (u8)
    [ EncodedRef ]   // only if has_ref == 0x01
  ```

  with:

  * `has_ref = 0x00` → no value present (no `EncodedRef` follows),
  * `has_ref = 0x01` → exactly one `EncodedRef` follows,
  * other `has_ref` values are encoding errors.

---

## 3. Descriptor Logical Model

### 3.1 DagSchemeDescriptor (logical)

At the logical level, the DAG scheme descriptor is modeled as:

```text
DagSchemeDescriptor {
  pel1_version:        uint16

  scheme_name:         String          // logical identifier, e.g. "PEL/PROGRAM-DAG/1"

  program_type_tag:    TypeTag         // TYPE_TAG_PEL_PROGRAM_DAG_1
  program_enc_profile: EncodingProfileId
                                        // PEL_ENC_PROGRAM_DAG_V1 (0x0101)

  trace_profile_ref:   optional Reference
                                        // to a trace-profile descriptor (e.g. PEL/TRACE-DAG-DESC/1)
  opreg_ref:           optional Reference
                                        // to a canonical operation-registry descriptor (e.g. OPREG/PEL1-KERNEL)
}
```

This structure is **logical**; the descriptor’s concrete bytes are defined in §4.

The `program_type_tag` / `program_enc_profile` pair tells engines **how to interpret Program Artifacts** for this scheme:

* `program_type_tag` ⇒ which `TypeTag` marks an Artifact as a DAG Program for this scheme;
* `program_enc_profile` ⇒ which Program encoding profile (specifically, `PEL_ENC_PROGRAM_DAG_V1` from `ENC/PEL-PROGRAM-DAG/1`) to use when decoding/encoding those Artifacts.

This pair is independent from the **ASL-level encoding profile** used for the descriptor Artifact itself, which is always `ASL_ENC_CORE_V1` when deriving `SchemeRef_DAG_1` (§5.2).

This document does not require engines to inspect all fields at runtime. In particular:

* engines that already know `SchemeRef_DAG_1`, `TYPE_TAG_PEL_PROGRAM_DAG_1`, and `PEL_ENC_PROGRAM_DAG_V1` MAY treat the descriptor as metadata; and
* the canonical binding to `PEL/PROGRAM-DAG/1` is via `SchemeRef_DAG_1` and the `program_type_tag` / `program_enc_profile` pair, not via `scheme_name` or the optional references.

### 3.2 Field semantics & invariants

For the **Amduat 2.0 baseline DAG scheme descriptor** (the one used to define `SchemeRef_DAG_1`) the following invariants apply:

* `pel1_version`:

  * MUST be `1`.
  * Indicates the descriptor is written against `PEL/1-CORE` version 1 semantics.
  * Future PEL versions MAY introduce new descriptor formats, but MUST NOT change the meaning of `pel1_version = 1`.

* `scheme_name`:

  * MUST be the UTF-8 string `"PEL/PROGRAM-DAG/1"`, compared byte-for-byte and case-sensitively.
  * Serves as a human-readable identifier and sanity check.
  * Changing `scheme_name` (while keeping other fields identical) changes the descriptor value and therefore its `Reference`.

* `program_type_tag`:

  * MUST equal the `TypeTag` used for DAG Program Artifacts, symbolically:

    ```text
    program_type_tag = TYPE_TAG_PEL_PROGRAM_DAG_1
    ```

  * The concrete numeric `tag_id` is assigned by the TypeTag registry and is not restated here.

* `program_enc_profile`:

  * MUST be the `EncodingProfileId` of `ENC/PEL-PROGRAM-DAG/1`, symbolically:

    ```text
    program_enc_profile = PEL_ENC_PROGRAM_DAG_V1
    ```

  * In the current encoding profile, this is `0x0101`, but this spec treats it symbolically; the numeric value comes from the encoding profile registry.

* `trace_profile_ref`:

  * Optionally points at a descriptor Artifact that defines a canonical trace profile for this scheme (e.g. a future `PEL/TRACE-DAG-DESC/1`).

  * For the **canonical Amduat 2.0 baseline descriptor used to define `SchemeRef_DAG_1`**, this field is **absent**:

    ```text
    trace_profile_ref = None
    ```

  * Future companion descriptors MAY set this field, but those will not be the descriptor used for `SchemeRef_DAG_1`.

* `opreg_ref`:

  * Optionally points at a descriptor Artifact for the canonical operation registry for this scheme (e.g. `OPREG/PEL1-KERNEL`).

  * For the **canonical Amduat 2.0 baseline descriptor used to define `SchemeRef_DAG_1`**, this field is **absent**:

    ```text
    opreg_ref = None
    ```

  * As with `trace_profile_ref`, future descriptors MAY set this field, but those will define distinct `SchemeRef` values, not `SchemeRef_DAG_1`.

### 3.3 Canonical descriptor value for the DAG scheme

Let:

```text
DagSchemeDescriptor_DAG_1 : DagSchemeDescriptor
```

denote the unique logical descriptor value that satisfies:

```text
DagSchemeDescriptor_DAG_1 {
  pel1_version        = 1

  scheme_name         = "PEL/PROGRAM-DAG/1"

  program_type_tag    = TYPE_TAG_PEL_PROGRAM_DAG_1
  program_enc_profile = PEL_ENC_PROGRAM_DAG_V1

  trace_profile_ref   = None
  opreg_ref           = None
}
```

All Amduat 2.0 baseline components MUST agree on this logical value as “the” DAG scheme descriptor for binding `PEL/PROGRAM-DAG/1` to a `SchemeRef`.

Any descriptor with different logical contents (including differing optional fields) describes either:

* a different scheme, or
* a different profile or version of the DAG scheme,

and MUST NOT be used to derive `SchemeRef_DAG_1`.

Because the descriptor is carried in an ASL/1 Artifact and scheme references are content-addressed, any change to this descriptor (even in optional fields) yields a different `Reference`; `SchemeRef_DAG_1` is therefore permanently tied to exactly this v1 descriptor.

---

## 4. Descriptor Artifact & Encoding

### 4.1 Descriptor TypeTag

The DAG scheme descriptor is carried by an ASL/1 Artifact with a dedicated TypeTag:

```text
TYPE_TAG_PEL_SCHEME_DESC_1 : TypeTag
```

* `TYPE_TAG_PEL_SCHEME_DESC_1` is a symbolic constant for a `TypeTag.tag_id` assigned by the global `TypeTag` registry.
* This `TypeTag` MUST be used for the DAG scheme descriptor Artifact defined in this document.
* Other PEL scheme descriptors MAY reuse this `TypeTag` and the encoding in §4.2, but such reuse MUST be specified in their own descriptor documents.

The descriptor Artifact for the DAG scheme has the shape:

```text
DagSchemeDescriptorArtifact_DAG_1 : Artifact {
  type_tag = TYPE_TAG_PEL_SCHEME_DESC_1
  bytes    = DagSchemeDescriptorBytes_DAG_1
}
```

where `DagSchemeDescriptorBytes_DAG_1` is the canonical encoding of `DagSchemeDescriptor_DAG_1` defined in §4.2.

This Artifact itself is encoded to `ArtifactBytes` using `ASL_ENC_CORE_V1` when deriving `SchemeRef_DAG_1` (§5.2). That use of `ASL_ENC_CORE_V1` is **independent** of the `program_enc_profile` field, which is about how Program Artifacts are encoded.

### 4.2 Descriptor encoding layout

The descriptor value `DagSchemeDescriptor` is encoded into `DagSchemeDescriptorBytes` using a small, fixed layout over primitive types:

```text
DagSchemeDescriptorBytes ::
  pel1_version        (u16)

  scheme_name         (Utf8String)

  program_type_tag    (u32)   // TypeTag.tag_id
  program_enc_profile (u16)   // EncodingProfileId

  has_trace_profile   (u8)
  [ trace_profile_ref (EncodedRef) ]   // if has_trace_profile == 0x01

  has_opreg_ref       (u8)
  [ opreg_ref         (EncodedRef) ]   // if has_opreg_ref == 0x01
```

Constraints:

* `pel1_version` MUST be `1` for descriptors governed by this document.

* `scheme_name` MUST be encoded as a `Utf8String` (§2.4).

* `program_type_tag` encodes `TypeTag.tag_id` for `TYPE_TAG_PEL_PROGRAM_DAG_1`.

* `program_enc_profile` encodes the `EncodingProfileId` for `PEL_ENC_PROGRAM_DAG_V1` as defined in `ENC/PEL-PROGRAM-DAG/1`.

* `has_trace_profile`:

  * `0x00` → `trace_profile_ref` is absent; no bytes for it follow.
  * `0x01` → `trace_profile_ref` is present and encoded as `EncodedRef`.
  * Any other value is an encoding error.

* `has_opreg_ref`:

  * `0x00` → `opreg_ref` is absent.
  * `0x01` → `opreg_ref` is present and encoded as `EncodedRef`.
  * Any other value is an encoding error.

For the **canonical baseline descriptor**:

```text
pel1_version        = 1
scheme_name         = "PEL/PROGRAM-DAG/1"
program_type_tag    = TYPE_TAG_PEL_PROGRAM_DAG_1.tag_id
program_enc_profile = PEL_ENC_PROGRAM_DAG_V1
has_trace_profile   = 0x00
has_opreg_ref       = 0x00
```

The resulting bytes are `DagSchemeDescriptorBytes_DAG_1`.

### 4.3 Encoding and decoding (normative)

Let:

```text
encode_dag_scheme_descriptor_v1 : DagSchemeDescriptor -> OctetString
decode_dag_scheme_descriptor_v1 : OctetString -> DagSchemeDescriptor | error
```

be the encoding and decoding functions for this profile.

**Encoding:**

Given a `DagSchemeDescriptor` value:

1. Emit `pel1_version (u16)`.

2. Emit `scheme_name` as `Utf8String`.

3. Emit `program_type_tag.tag_id` as `u32`.

4. Emit `program_enc_profile` as `u16`.

5. For `trace_profile_ref`:

   * If absent:

     * emit `has_trace_profile = 0x00`.

   * If present:

     * emit `has_trace_profile = 0x01`,
     * emit `trace_profile_ref` as `EncodedRef` (u32 length + `ReferenceBytes`).

6. For `opreg_ref`:

   * If absent:

     * emit `has_opreg_ref = 0x00`.

   * If present:

     * emit `has_opreg_ref = 0x01`,
     * emit `opreg_ref` as `EncodedRef`.

The concatenation of all emitted bytes is `DagSchemeDescriptorBytes`.

**Decoding:**

Given an `OctetString` known to contain exactly one `DagSchemeDescriptorBytes` value:

1. Read `pel1_version (u16)`.

   * If `pel1_version != 1`, decoders that only implement this version MUST treat this as an encoding error. Future decoders MAY support additional versions but MUST NOT reinterpret `pel1_version = 1`.

2. Read `scheme_name` as `Utf8String` and validate UTF-8.

3. Read `program_type_tag (u32)` and construct `TypeTag { tag_id = program_type_tag }`.

4. Read `program_enc_profile (u16)`.

5. Read `has_trace_profile (u8)`:

   * `0x00` → `trace_profile_ref = None`.
   * `0x01` → read `trace_profile_ref` as `EncodedRef` and decode to `Reference`.
   * other values → encoding error.

6. Read `has_opreg_ref (u8)`:

   * `0x00` → `opreg_ref = None`.
   * `0x01` → read `opreg_ref` as `EncodedRef` and decode to `Reference`.
   * other values → encoding error.

7. If there are trailing bytes after successfully decoding all fields, decoders MAY treat this as an encoding error in contexts expecting a single `DagSchemeDescriptorBytes` value.

The mapping between `DagSchemeDescriptor` and `DagSchemeDescriptorBytes` defined here is the only encoding/decoding pair permitted by this profile.

### 4.4 Canonicality and injectivity

The mapping:

```text
DagSchemeDescriptor -> DagSchemeDescriptorBytes
```

defined above MUST be:

* **Injective** — distinct logical descriptor values MUST produce distinct byte strings.
* **Stable** — the same logical descriptor MUST always encode to the same bytes across implementations and time.
* **Streaming-friendly** — encoders and decoders MUST be implementable in a single forward pass; all length prefixes appear before their content.

In particular:

* Field ordering MUST NOT vary.
* No alternative encodings (e.g. different integer widths, string formats, or positions) are permitted.
* `EncodedRef` MUST always use canonical `ReferenceBytes` from `ENC/ASL1-CORE v1` (and therefore, when `HASH/ASL1` is present, inherit its digest-length consistency checks).

For the canonical DAG scheme, `DagSchemeDescriptor_DAG_1` encodes to a single, immutable `DagSchemeDescriptorBytes_DAG_1`. That byte sequence MUST NOT change once published.

### 4.5 Error handling (encoding layer)

Decoders for this profile MUST treat as **encoding errors** at the descriptor level:

1. **Unsupported `pel1_version`**

   * `pel1_version != 1`.

2. **Invalid presence flags**

   * `has_trace_profile` or `has_opreg_ref` not in `{ 0x00, 0x01 }`.

3. **Invalid `EncodedRef`**

   * `ref_len < 2`, or
   * insufficient bytes to read `ref_bytes`, or
   * `ref_bytes` cannot be decoded as `ReferenceBytes` under `ENC/ASL1-CORE v1` (including digest-length mismatches for known `hash_id`s when `HASH/ASL1` is implemented).

4. **Invalid `Utf8String` for `scheme_name`**

   * `scheme_name` bytes are not valid UTF-8.

5. **Truncation and over-long payloads**

   * Not enough bytes to read any declared field.
   * In contexts expecting a single descriptor value, additional trailing bytes after decoding all fields.

Mapping from these encoding errors to external error codes (e.g. `ERR_PEL_SCHEME_DESC_INVALID`) is implementation-specific.

---

## 5. Definition of `SchemeRef_DAG_1`

### 5.1 General derivation from the descriptor Artifact

Let `DagSchemeDescriptorArtifact` be any Artifact of the form:

```text
Artifact {
  type_tag = TYPE_TAG_PEL_SCHEME_DESC_1
  bytes    = DagSchemeDescriptorBytes
}
```

such that `DagSchemeDescriptorBytes` decodes to some `DagSchemeDescriptor` value via `decode_dag_scheme_descriptor_v1`.

Given:

* encoding profile `P = ASL_ENC_CORE_V1`,
* hash algorithm `H` with `HashId = HID`,

the scheme reference for this descriptor under `(P, H)` is:

```text
ArtifactBytes = encode_artifact_core_v1(DagSchemeDescriptorArtifact)
digest        = H(ArtifactBytes)
SchemeRef     = Reference { hash_id = HID, digest = digest }
```

This is `ASL/CORE-REF-DERIVE/1` instantiated with `ASL_ENC_CORE_V1`.

### 5.2 Canonical `SchemeRef_DAG_1` for Amduat 2.0

For the Amduat 2.0 baseline DAG scheme, we choose:

* `DagSchemeDescriptorArtifact = DagSchemeDescriptorArtifact_DAG_1`,
* `P = ASL_ENC_CORE_V1` (EncodingProfileId = 0x0001),
* `H = HASH-ASL1-256` (the mandatory ASL1-256 algorithm),
* `HashId(H) = 0x0001`.

Then:

```text
ArtifactBytes_DAG_1 = encode_artifact_core_v1(DagSchemeDescriptorArtifact_DAG_1)
digest_DAG_1        = HASH-ASL1-256(ArtifactBytes_DAG_1)
SchemeRef_DAG_1     = Reference { hash_id = 0x0001, digest = digest_DAG_1 }
```

In the Amduat 2.0 baseline:

* `PEL/PROGRAM-DAG/1` binds its scheme symbol:

  ```text
  SchemeRef = SchemeRef_DAG_1
  Exec_s    = Exec_DAG
  ```

  to this `Reference`.

* Engines that implement the Amduat 2.0 baseline DAG scheme MUST treat this specific `Reference` as “the” scheme identifier for `PEL/PROGRAM-DAG/1`.

### 5.3 Alternative deployments (non-baseline)

Other deployments MAY derive alternative scheme references from:

* the same descriptor Artifact but a different `(EncodingProfileId, HashId)` pair, or
* a different descriptor Artifact (e.g. one with additional optional references or different metadata).

Such configurations:

* are not the Amduat 2.0 baseline, and
* MUST NOT reuse the name `SchemeRef_DAG_1` for any scheme reference other than the one defined in §5.2.

Profiles MAY define their own symbolic names (e.g. `SchemeRef_DAG_1_PQ`) for alternate `(EncodingProfileId, HashId)` combinations, but those are outside the scope of this document.

---

## 6. Layering & Usage

### 6.1 Engine behavior

A PEL/1-CORE engine that claims support for the DAG scheme in the Amduat 2.0 baseline:

* MUST recognize `SchemeRef_DAG_1` (as defined in §5.2) as the scheme identifier corresponding to `PEL/PROGRAM-DAG/1`.

* MUST implement:

  ```text
  Exec_s = Exec_DAG
  ```

  when invoked with `scheme_ref = SchemeRef_DAG_1`.

* MUST treat Program Artifacts for this scheme as ASL/1 Artifacts whose `type_tag` equals the `program_type_tag` from the descriptor (i.e. `TYPE_TAG_PEL_PROGRAM_DAG_1`). The `program_enc_profile` field gives the encoding profile ID that engines use to select the appropriate Program encoding/decoding logic; in the Amduat 2.0 baseline this ID is `PEL_ENC_PROGRAM_DAG_V1` as defined in `ENC/PEL-PROGRAM-DAG/1`.

* MUST treat the descriptor Artifact as immutable metadata. Engines MAY bake in `SchemeRef_DAG_1` and the associated `TypeTag` and profile IDs at build time; they are not required to fetch or decode the descriptor on the execution hot path.

Nothing in this document requires engines to use `trace_profile_ref` or `opreg_ref` at runtime. Those fields exist so that tooling and profiles can discover related descriptors in a stable, content-addressed way.

### 6.2 Configuration and discovery

Tooling and configuration systems MAY use the descriptor to:

* discover:

  * which `TypeTag` is used for DAG Programs,
  * which encoding profile ID applies to those Programs,
  * optional references to trace and operation-registry descriptors;

* populate engine configuration tables, e.g.:

  ```text
  scheme_table[SchemeRef_DAG_1] = {
    program_type_tag    = TYPE_TAG_PEL_PROGRAM_DAG_1,
    program_enc_profile = PEL_ENC_PROGRAM_DAG_V1,
    trace_profile_ref   = None,
    opreg_ref           = None,
  }
  ```

This document:

* MUST NOT introduce any store-level behavior, TGK edge semantics, certification rules, fact semantics, or overlay semantics.
* ONLY binds PEL-level scheme semantics (`Exec_DAG`) to ASL-level values (`Artifact`, `TypeTag`, `Reference`).

---

## 7. Conformance

An implementation is **PEL/PROGRAM-DAG-DESC/1–conformant** if it satisfies all of:

1. **Descriptor encoding & decoding**

   * Implements `encode_dag_scheme_descriptor_v1` / `decode_dag_scheme_descriptor_v1` as in §4.2–§4.3.
   * Treats `pel1_version = 1` as the only supported version for this descriptor profile.
   * Enforces presence flags and `EncodedRef` structure as defined in §2.4 and §4.5.

2. **Recognition of the descriptor Artifact**

   * Recognizes `TYPE_TAG_PEL_SCHEME_DESC_1` as the PEL scheme descriptor tag for this document.
   * For `type_tag = TYPE_TAG_PEL_SCHEME_DESC_1`, is able to decode `bytes` as `DagSchemeDescriptorBytes` and obtain a `DagSchemeDescriptor`.
   * Recognizes `DagSchemeDescriptor_DAG_1` (per §3.3) as the canonical descriptor value for the DAG scheme in Amduat 2.0 (e.g. by comparing decoded fields).

3. **Derivation of `SchemeRef_DAG_1`**

   * When configured with `ASL_ENC_CORE_V1` and `HASH-ASL1-256` (`HashId = 0x0001`), derives `SchemeRef_DAG_1` exactly as in §5.2 from `DagSchemeDescriptorArtifact_DAG_1`.
   * Does not derive `SchemeRef_DAG_1` from any other Artifact or under any other `(EncodingProfileId, HashId)` pair.

4. **Scheme binding**

   * For `SchemeRef = SchemeRef_DAG_1`, implements `Exec_s` as `Exec_DAG` per `PEL/PROGRAM-DAG/1`.
   * Uses `program_type_tag` and `program_enc_profile` from the descriptor when interpreting Program Artifacts for this scheme (at build time or configuration time; not necessarily on every execution).

5. **Layering discipline**

   * Does not introduce store, TGK, certificate, fact, or overlay semantics into the descriptor logic.
   * Treats this document purely as a binding between:

     * the PEL scheme (`PEL/PROGRAM-DAG/1`),
     * its program representation (`TYPE_TAG_PEL_PROGRAM_DAG_1` + `PEL_ENC_PROGRAM_DAG_V1`),
     * and an ASL/1 `SchemeRef` (`SchemeRef_DAG_1`).

Everything else — store wiring, TGK edges, receipts, facts, overlays, and domain semantics — is delegated to other specifications.

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## 8. Version History

* **0.1.6 — 2025-11-16** — Marked `ENC/PEL-PROGRAM-DAG/1` as a normative dependency and clarified that `program_enc_profile` is specifically `PEL_ENC_PROGRAM_DAG_V1`.
* **0.1.5 — 2025-11-16** — Clarified the role of `program_enc_profile` vs `ASL_ENC_CORE_V1` and tightened wording around how Program Artifacts are interpreted.
* **0.1.4 — 2025-11-16** — Tightened decoding wording, clarified the uniqueness of the encoding/decoding pair, and polished streaming/injectivity language.
* **0.1.3 — 2025-11-16** — Clarified RFC 2119 usage, layering around program encoding, and tightened conformance wording.
* **0.1.2 — 2025-11-16** — Refined encoding error rules, clarified `TYPE_TAG_PEL_SCHEME_DESC_1` reuse, and described engine usage more explicitly.
* **0.1.1 — 2025-11-16** — Added logical descriptor model, descriptor Artifact & encoding, canonical descriptor value, and explicit `SchemeRef_DAG_1` derivation.
* **0.1.0 — 2025-11-16** — Initial skeleton with structure, roles, and high-level field model for the DAG scheme descriptor.

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## Document History

* **0.1.6 (2025-11-16):** Registered as Tier-1 spec and aligned to the Amduat 2.0 substrate baseline.