This PR fixes a regression in `Sym.simp` where rewrite rules whose LHS
contains a lambda over a pattern variable (e.g. `∃ x, a = x`) failed to
match targets with semantically equivalent structure.
`Sym.etaReduceAux` previously refused any eta-reduction whenever the
body had loose bound variables, but patterns produced by stripping outer
foralls always carry such loose bvars. The eta-reduction therefore
skipped patterns while still firing on the target, producing mismatched
discrimination tree keys and no match.
The fix narrows the check to loose bvars in the range `[0, n)` (those
that would actually refer to the peeled binders) and lowers any
remaining loose bvars by `n` so that pattern-variable references stay
consistent in the reduced expression. The discrimination tree now
classifies patterns like `exists_eq_True : (∃ x, a = x) = True` with
their full structure rather than falling back to `.other`.
Includes a regression test (`sym_simp_1.lean`) and Sebastian Graf's MWE
(`sym_eta_mwe.lean`).
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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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