exact Nat.le_of_lt hi
case succ.refl.hi
α : Type u_1
n : Nat
f : Fin (Nat.succ n) → α
i : Nat
ih :
  i ≤ n →
    ∀ (o : Array α),
      size o = n - i →
        Std.Range.forIn'.loop 0 (Nat.succ n) 1 (fun i h r => ForInStep.yield (push r (f { val := i, isLt := ⋯ })))
            (Nat.succ i) (n - i) ⋯ o =
          push
            (Std.Range.forIn'.loop 0 n 1 (fun i h r => ForInStep.yield (push r (f { val := i, isLt := ⋯ }))) i (n - i) ⋯
              o)
            (f { val := n, isLt := ⋯ })
hi : Nat.succ i ≤ n
o : Array α
ho : size o = n - Nat.succ i
hn : n - Nat.succ i < n
hn' : n - Nat.succ i < Nat.succ n
this : n - Nat.succ i + 1 = n - i
⊢ i ≤ n

case succ.refl.ho
α : Type u_1
n : Nat
f : Fin (Nat.succ n) → α
i : Nat
ih :
  i ≤ n →
    ∀ (o : Array α),
      size o = n - i →
        Std.Range.forIn'.loop 0 (Nat.succ n) 1 (fun i h r => ForInStep.yield (push r (f { val := i, isLt := ⋯ })))
            (Nat.succ i) (n - i) ⋯ o =
          push
            (Std.Range.forIn'.loop 0 n 1 (fun i h r => ForInStep.yield (push r (f { val := i, isLt := ⋯ }))) i (n - i) ⋯
              o)
            (f { val := n, isLt := ⋯ })
hi : Nat.succ i ≤ n
o : Array α
ho : size o = n - Nat.succ i
hn : n - Nat.succ i < n
hn' : n - Nat.succ i < Nat.succ n
this : n - Nat.succ i + 1 = n - i
⊢ size (push o (f { val := n - Nat.succ i, isLt := ⋯ })) = n - i
case succ.refl.ho
α : Type u_1
n : Nat
f : Fin (Nat.succ n) → α
i : Nat
ih :
  i ≤ n →
    ∀ (o : Array α),
      size o = n - i →
        Std.Range.forIn'.loop 0 (Nat.succ n) 1 (fun i h r => ForInStep.yield (push r (f { val := i, isLt := ⋯ })))
            (Nat.succ i) (n - i) ⋯ o =
          push
            (Std.Range.forIn'.loop 0 n 1 (fun i h r => ForInStep.yield (push r (f { val := i, isLt := ⋯ }))) i (n - i) ⋯
              o)
            (f { val := n, isLt := ⋯ })
hi : Nat.succ i ≤ n
o : Array α
ho : size o = n - Nat.succ i
hn : n - Nat.succ i < n
hn' : n - Nat.succ i < Nat.succ n
this : n - Nat.succ i + 1 = n - i
⊢ size (push o (f { val := n - Nat.succ i, isLt := ⋯ })) = n - i