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This PR adds a `result? : Option TraceResult` field to `TraceData` and populates it in `withTraceNode` and `withTraceNodeBefore`, so that metaprograms walking trace trees can determine success/failure structurally instead of string-matching on emoji. `TraceResult` has three cases: `.success` (checkEmoji), `.failure` (crossEmoji), and `.error` (bombEmoji, exception thrown). An `ExceptToTraceResult` typeclass converts `Except` results to `TraceResult` directly, with instances for `Bool` and `Option`. `TraceResult.toEmoji` converts back to emoji for display. This replaces the previous `ExceptToEmoji` typeclass — `TraceResult` is now the primary representation rather than being derived from emoji strings. `withTraceNodeBefore` (used by `isDefEq`) uses `ExceptToTraceResult.toTraceResult` directly, correctly handling `Bool` (`.ok false` = failure) and `Option` (`.ok none` = failure), with `Except.error` mapping to `.error`. For `withTraceNode`, `result?` defaults to `none`. Callers can pass `mkResult?` to provide structured results; when set, the corresponding emoji is auto-prepended to the message. Motivated by mathlib's `#defeq_abuse` diagnostic tactic (https://github.com/leanprover-community/mathlib4/pull/35750) which currently string-matches on emoji to determine trace node outcomes. See https://leanprover.zulipchat.com/#narrow/channel/113488-general/topic/backward.2EisDefEq.2ErespectTransparency 🤖 Prepared with Claude Code --------- Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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