rw [←specSubtype]
n : Nat
s : Setoid (Fin n)
inst✝ : DecidableRel Setoid.r
k : Fin (quot s)
i : Fin n
h✝ : Quotient.mk s (decodeSubtype (repr s) k).val = Quotient.mk s i
h : (decodeSubtype (repr s) k).val ≈ i
j : Fin n
hj : (Fin.find? fun b => decide (Setoid.r i b)) = some j
⊢ (let_fun this := ⋯;
    encodeSubtype (repr s) ⟨j, this⟩) =
    k
n : Nat
s : Setoid (Fin n)
inst✝ : DecidableRel Setoid.r
k : Fin (quot s)
i : Fin n
h✝ : Quotient.mk s (decodeSubtype (repr s) k).val = Quotient.mk s i
h : (decodeSubtype (repr s) k).val ≈ i
j : Fin n
hj : (Fin.find? fun b => decide (Setoid.r i b)) = some j
⊢ decodeSubtype (repr s) k = ⟨j, ⋯⟩