Lean 4 Mathlib: rw fails after comap because Finset.filter decidability instances differ

Pull back a graph along an embedding with G.comap g, then turn the edge count into sums of #{v | G.Adj ..}. The filter in the resulting hypothesis carries a decidability instance built from comap (fun a => inst (Sum.inl (e a)) ...), while your goal uses the classical one. rw [h] at hres then fails with 'motive is not type correct' or 'did not find pattern', even though both sides print identically. Fix: state the target inequality yourself and close it with convert hres with w followed by exact filter_congr_decidable _ _ _. For goals of the form #{..} = #{..} produced by convert this using 4, the same <;> exact filter_congr_decidable _ _ _ works. simpa also fails here, because it only reports a type mismatch after simplification.