mail-threading 0.1.3

JWZ email threading (RFC 5256). Reconstruct message threads from References/In-Reply-To headers, with subject fallback for broken clients.
Documentation 
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{
  "name": "subject-merge-disabled",
  "description": "Disabling subject fallback keeps headerless same-subject messages separate.",
  "spec": {
    "source": "public API contract",
    "url": "https://www.jwz.org/doc/threading.html",
    "behavior": "subject fallback is configurable"
  },
  "options": {
    "subject_merge": false
  },
  "input": [
    {
      "id": "<strict-subject-root@example>",
      "message_id": "<strict-subject-root@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "Strict",
      "date": "2026-05-15T09:00:00Z"
    },
    {
      "id": "<strict-subject-reply@example>",
      "message_id": "<strict-subject-reply@example>",
      "in_reply_to": null,
      "references": [],
      "subject": "Re: Strict",
      "date": "2026-05-15T09:05:00Z"
    }
  ],
  "expected": {
    "threads": [
      {
        "root": "<strict-subject-root@example>",
        "messages": [
          "<strict-subject-root@example>"
        ]
      },
      {
        "root": "<strict-subject-reply@example>",
        "messages": [
          "<strict-subject-reply@example>"
        ]
      }
    ]
  }
}