dsimp at *
α : Type u
n : Nat
x : SizedArray α n
p : α → Bool
stop : Nat
hstop : stop ≤ n
i : Nat
x✝ : findIdx.loop x p stop hstop i < n
hi : i < stop
h :
  (let_fun this := (_ : i < n);
    bif p x[i] then i else findIdx.loop x p stop hstop (i + 1)) <
    stop
⊢ p
      x[let_fun this := (_ : i < n);
        bif p x[i] then i else findIdx.loop x p stop hstop (i + 1)] =
    true
α : Type u
n : Nat
x : SizedArray α n
p : α → Bool
stop : Nat
hstop : stop ≤ n
i : Nat
x✝ : findIdx.loop x p stop hstop i < n
hi : i < stop
h : (bif p x[i] then i else findIdx.loop x p stop hstop (i + 1)) < stop
⊢ p x[bif p x[i] then i else findIdx.loop x p stop hstop (i + 1)] = true