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This PR optimizes the `String.reduceEq`, `String.reduceNe`, and `Sym.Simp` string equality simprocs to produce kernel-efficient proofs. Previously, these used `String.decEq` which forced the kernel to run UTF-8 encoding/decoding and byte array comparison, causing 86+ kernel unfoldings on short strings. The new approach reduces string inequality to `List Char` via `String.ofList_injective`, then uses two strategies depending on the difference: - **Different characters at position `i`**: Projects to `Nat` via `congrArg (fun l => (List.get!Internal l i).toNat)`, then uses `Nat.ne_of_beq_eq_false rfl`. This avoids `Decidable` instances entirely — the kernel only evaluates `Nat.beq` on two concrete natural numbers. - **One string is a prefix of the other**: Uses `congrArg (List.drop n ·)` with `List.cons_ne_nil`, which is a definitional proof requiring no `decide` step at all. For equal strings, `eq_true rfl` avoids kernel evaluation entirely. The shared proof construction is in `Lean.Meta.mkStringLitNeProof` (`Lean/Meta/StringLitProof.lean`), used by both the standard simprocs and the `Sym.Simp` ground evaluator. Kernel max unfolds for `"hello" ≠ "foo"`: 86+ → 6. --------- Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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