diff --git a/HumanEvalLean/HumanEval123.lean b/HumanEvalLean/HumanEval123.lean
index b7e1b2b..afa837f 100644
--- a/HumanEvalLean/HumanEval123.lean
+++ b/HumanEvalLean/HumanEval123.lean
@@ -1,5 +1,430 @@
-def get_odd_collatz : Unit :=
-  ()
+import Std
+
+open Std
+
+/-!
+# HumanEval 123: List the odd numbers in a Collatz sequence in ascending order
+
+This problem asks us to return the odd numbers in a Collatz sequence. Since the Collatz
+conjecture is unsolved, we cannot prove termination for all inputs. This file demonstrates
+two approaches to handling this:
+
+1. Require a termination proof as an argument, proving that the Collatz sequence for the given input
+   reaches `1` eventually (guaranteed termination).
+2. Don't require a termination proof for calling the function, only for verification.
+-/
+
+/-!
+## Potentially missing API
+
+This section provides declarations that might be added to the standard library.
+-/
+
+theorem Acc.invTransGen {x y : α} (h₁ : Acc r x) (h₂ : Relation.TransGen r y x) : Acc r y := by
+  simpa [acc_transGen_iff] using h₁.transGen.inv h₂
+
+theorem Std.compare_ne_eq [Ord α] [LawfulEqOrd α] {x y : α} :
+    compare x y ≠ .eq ↔ x ≠ y := by
+  simp
+
+instance : LawfulOrderOrd Nat where
+  isLE_compare := by grind [Nat.isLE_compare]
+  isGE_compare := by grind [Nat.isGE_compare]
+
+attribute [grind =] TreeSet.mem_toList
+
+/-!
+## Preliminaries
+
+We start by defining what it means to make a step in the Collatz sequence.
+-/
+
+/--
+Only valid if called for `n > 1`.
+-/
+def collatzStep (n : Nat) : Nat :=
+    if n % 2 = 0 then n / 2 else n * 3 + 1
+
+/--
+`CollatzRel a b` signifies that `b` is a valid successor of `a` in a Collatz sequence.
+Here, we assume the sequence stops at `1`, so `1` has no successor.
+-/
+def CollatzRel : Nat → Nat → Prop := fun m n =>
+    1 < n ∧ collatzStep n = m
+
+/-!
+## Implementation 1: guaranteed to terminate
+
+Next, we provide an implementation using well-founded recursion. `oddCollatz₁ n` is guaranteed to
+terminate, but calling it requires a proof that the Collatz sequence for `n` is finite.
+-/
+
+instance : WellFoundedRelation { m : Nat // Acc CollatzRel m } := Acc.wfRel
+
+/--
+As an optional improvement, we will implement a tactic below that automatically discharges the
+termination proof obligation, making is easier to call our solution. We declare the syntax first
+and provide the implementation later.
+-/
+syntax "try_decide" : tactic
+
+def oddCollatz₁ (n : Nat) (h : Acc CollatzRel n := by try_decide) : List Nat :=
+  (collectOddCollatz ⟨n, h⟩ ∅).toList
+where
+  -- We attach a proof that `1` is reachable from `n` in finitely many steps to ensure termination.
+  collectOddCollatz (n : { n : Nat // Acc CollatzRel n }) (acc : TreeSet Nat compare) :
+      TreeSet Nat compare :=
+    if h : n.val > 1 then
+      collectOddCollatz ⟨collatzStep n, n.property.inv (by grind [CollatzRel])⟩
+        (if n.val % 2 = 0 then acc else acc.insert n.val)
+    else if n.val = 1 then
+      acc.insert 1
+    else
+      acc
+  termination_by n
+  decreasing_by
+    grind [CollatzRel]
+
+/-!
+### Optional: Implementing the `try_decide` tactic
+
+In order to make `oddCollatz` easier to use, we provide a tactic that automatically
+proves termination for a given input. The tactic `try_decide` will do so as long as the
+Collatz sequence is short enough.
+-/
+
+theorem collatzRel_collatzStep {n : Nat} (h : n > 1) :
+    CollatzRel (collatzStep n) n := by
+  grind [CollatzRel]
+
+theorem acc_collatzRel_collatzStep_iff {n : Nat} (h : n > 1) :
+    Acc CollatzRel (collatzStep n) ↔ Acc CollatzRel n := by
+  apply Iff.intro
+  · exact fun h => ⟨_, fun m hm => by grind [CollatzRel]⟩
+  · exact fun h => by grind [Acc.inv, collatzRel_collatzStep]
+
+def tryDecideTermination (n : Nat) (fuel : Nat) (h : Acc CollatzRel n ↔ P) :
+    Option (Decidable P) := do
+  match fuel with
+  | 0 => none
+  | fuel + 1 => do
+    if hn : n > 1 then
+      have := acc_collatzRel_collatzStep_iff hn
+      tryDecideTermination (collatzStep n) fuel (this.trans h)
+    else
+      return .isTrue (h.mp ⟨_, fun m hm => by grind [CollatzRel]⟩)
+
+def extractProof (d : Option (Decidable P)) : Option (PLift P) := do
+  match ← d with
+  | .isTrue h => return .up h
+  | .isFalse _ => none
+
+macro_rules
+  | `(tactic| try_decide)  =>
+    `(tactic| exact ((extractProof (tryDecideTermination _ 100 Iff.rfl)).get (by decide)).down)
+
+example : Acc CollatzRel 10 := by try_decide
+
+/-!
+## Tests for `oddCollatz₁`
+
+Observe that while `oddCollatz₁` is guaranteed to terminate, we do not need to manually supply
+the termination proofs because of the automatic use of our `try_decide` tactic.
+-/
+
+example : oddCollatz₁ 14 = [1, 5, 7, 11, 13, 17] := by native_decide
+example : oddCollatz₁ 5 = [1, 5] := by native_decide
+example : oddCollatz₁ 12 = [1, 3, 5] := by native_decide
+example : oddCollatz₁ 1 = [1] := by native_decide
+
+/-!
+## Verification of `oddCollatz₁`
+-/
+
+theorem oddCollatz₁_pairwise_distinct {n : Nat} {h : Acc CollatzRel n} :
+    (oddCollatz₁ n h).Pairwise (· ≠ ·) := by
+  simpa [oddCollatz₁] using TreeSet.distinct_toList (α := Nat) (cmp := compare)
+
+theorem oddCollatz₁_pairwise_lt {n : Nat} {h : Acc CollatzRel n} :
+    (oddCollatz₁ n h).Pairwise (· < ·) := by
+  simpa [oddCollatz₁, compare_eq_lt] using TreeSet.ordered_toList (α := Nat) (cmp := compare)
+
+theorem mod_two_eq_one_of_mem_oddCollatz₁ {m n : Nat} {h : Acc CollatzRel n} (hm : m ∈ oddCollatz₁ n h) :
+    m % 2 = 1 := by
+  simp only [oddCollatz₁, TreeSet.mem_toList] at hm
+  generalize (⟨n, h⟩ : Subtype _) = n at hm
+  generalize hg : (∅ : TreeSet Nat) = acc at hm
+  have hm' (k : Nat) : k ∈ acc → k % 2 = 1 := by simp [← hg]
+  clear hg
+  fun_induction oddCollatz₁.collectOddCollatz n acc <;> grind
+
+theorem transGen_collatzRel_of_mem_oddCollatz₁ {m n : Nat} {h : Acc CollatzRel n} (hm : m ∈ oddCollatz₁ n h)
+    (hne : m ≠ n) :
+    Relation.TransGen CollatzRel m n := by
+  simp only [oddCollatz₁, TreeSet.mem_toList] at hm
+  generalize htmp : (⟨n, h⟩ : Subtype _) = s at hm
+  rw [show n = s.val by grind] at hne ⊢
+  clear htmp
+  generalize hg : (∅ : TreeSet Nat) = acc at hm
+  have hm' (k : Nat) : k ∈ acc → k ≠ s → Relation.TransGen CollatzRel k s := by simp [← hg]
+  clear hg
+  generalize htmp : s = n₀ at hm' hne ⊢
+  have hs : s = n₀ ∨ Relation.TransGen CollatzRel s n₀ := Or.inl htmp
+  clear htmp
+  fun_induction oddCollatz₁.collectOddCollatz s acc
+  · rename_i n' acc' h' ih
+    apply ih hm
+    · grind
+    · apply Or.inr
+      rcases hs with rfl | hs
+      · exact .single (collatzRel_collatzStep (by grind))
+      · refine .trans ?_ hs
+        exact .single (collatzRel_collatzStep (by grind))
+  · grind
+  · grind
+
+theorem mem_collectOddCollatz_of_mem {n : { n : Nat // Acc CollatzRel n }} {acc : TreeSet Nat}
+    {m : Nat} (h : m ∈ acc) :
+    m ∈ oddCollatz₁.collectOddCollatz n acc := by
+  fun_induction oddCollatz₁.collectOddCollatz n acc <;> grind
+
+theorem mem_self_collectOddCollatz {n : { n : Nat // Acc CollatzRel n }} {acc : TreeSet Nat}
+    (h : n.val % 2 = 1) :
+    n.val ∈ oddCollatz₁.collectOddCollatz n acc := by
+  fun_cases oddCollatz₁.collectOddCollatz n acc <;> grind [mem_collectOddCollatz_of_mem]
+
+theorem mem_self_oddCollatz₁ {n : Nat} {h : Acc CollatzRel n} (h' : n % 2 = 1) :
+    n ∈ oddCollatz₁ n h := by
+  grind [oddCollatz₁, mem_self_collectOddCollatz]
+
+theorem collectOddCollatz_mono {n : { n : Nat // Acc CollatzRel n }} {acc' acc : TreeSet Nat}
+    (h : ∀ x, x ∈ acc' → x ∈ acc) {x : Nat} (hx : x ∈ oddCollatz₁.collectOddCollatz n acc') :
+    x ∈ oddCollatz₁.collectOddCollatz n acc := by
+  fun_induction oddCollatz₁.collectOddCollatz n acc generalizing acc' <;>
+    grind [oddCollatz₁.collectOddCollatz]
+
+theorem mem_oddCollatz₁_of_mem_oddCollatz₁_of_collatzRel {k m n : Nat} {hm hn}
+    (hmem : k ∈ oddCollatz₁ m hm) (hrel : CollatzRel m n) :
+    k ∈ oddCollatz₁ n hn := by
+  grind [oddCollatz₁, CollatzRel, oddCollatz₁.collectOddCollatz, collectOddCollatz_mono]
+
+theorem mem_oddCollatz₁_of_mem_oddCollatz₁_of_transGen {k m n : Nat} {hn}
+    (hrel : Relation.TransGen CollatzRel m n) (hmem : k ∈ oddCollatz₁ m (hn.invTransGen hrel)) :
+    k ∈ oddCollatz₁ n hn := by
+  induction hrel
+  · grind [mem_oddCollatz₁_of_mem_oddCollatz₁_of_collatzRel]
+  · grind [Acc.inv, mem_oddCollatz₁_of_mem_oddCollatz₁_of_collatzRel]
+
+theorem mem_oddCollatz₁_of_transGen {m n : Nat} {hn : Acc CollatzRel n}
+    (h : Relation.TransGen CollatzRel m n) (h' : m % 2 = 1) :
+    m ∈ oddCollatz₁ n hn := by
+  grind [mem_oddCollatz₁_of_mem_oddCollatz₁_of_transGen, mem_self_oddCollatz₁]
+
+theorem mem_oddCollatz₁_iff {m n : Nat} {h : Acc CollatzRel n} :
+    m ∈ oddCollatz₁ n h ↔ m % 2 = 1 ∧ (m = n ∨ Relation.TransGen CollatzRel m n) := by
+  grind [mod_two_eq_one_of_mem_oddCollatz₁, transGen_collatzRel_of_mem_oddCollatz₁,
+    mem_self_oddCollatz₁, mem_oddCollatz₁_of_transGen]
+
+/-!
+## Preparations for the second approach: more potentially missing API
+
+We need an improved version of the `extrinsicFix₂` fixpoint combinator in order to demonstrate
+the second solution.
+-/
+
+section Extrinsic
+open Relation
+
+variable {α : Sort _} {β : α → Sort _} {γ : (a : α) → β a → Sort _}
+  {C : α → Sort _} {C₂ : (a : α) → β a → Sort _} {C₃ : (a : α) → (b : β a) → γ a b → Sort _}
+
+@[inline]
+public def WellFounded.partialExtrinsicFix [∀ a, Nonempty (C a)] (R : α → α → Prop)
+    (F : ∀ a, (∀ a', R a' a → C a') → C a) (a : α) : C a :=
+  extrinsicFix (α := { a' : α // a' = a ∨ TransGen R a' a }) (C := (C ·.1))
+      (fun p q => R p.1 q.1)
+      (fun a recur => F a.1 fun a' hR => recur ⟨a', by
+        cases a.property
+        · grind [TransGen.single]
+        · apply Or.inr
+          apply TransGen.trans ?_ ‹_›
+          apply TransGen.single
+          assumption⟩ ‹_›) ⟨a, Or.inl rfl⟩
+
+public theorem WellFounded.extrinsicFix_invImage {α' : Sort _} [∀ a, Nonempty (C a)] (R : α → α → Prop) (f : α' → α)
+    (F : ∀ a, (∀ a', R a' a → C a') → C a) (F' : ∀ a, (∀ a', R (f a') (f a) → C (f a')) → C (f a))
+    (h : ∀ a r, F (f a) r = F' a fun a' hR => r (f a') hR) (a : α') (h : WellFounded R) :
+    extrinsicFix (C := (C <| f ·)) (InvImage R f) F' a = extrinsicFix (C := C) R F (f a) := by
+  have h' := h
+  rcases h with ⟨h⟩
+  specialize h (f a)
+  have : Acc (InvImage R f) a := InvImage.accessible _ h
+  clear h
+  induction this
+  rename_i ih
+  rw [extrinsicFix_eq_apply, extrinsicFix_eq_apply, h]
+  · congr; ext a x
+    rw [ih _ x]
+  · assumption
+  · exact InvImage.wf _ ‹_›
+
+public theorem WellFounded.partialExtrinsicFix_eq [∀ a, Nonempty (C a)] (R : α → α → Prop)
+    (F : ∀ a, (∀ a', R a' a → C a') → C a) (a : α) (h : Acc R a) :
+    partialExtrinsicFix R F a = F a (fun a' _ => partialExtrinsicFix R F a') := by
+  simp only [partialExtrinsicFix]
+  rw [extrinsicFix_eq_apply]
+  congr; ext a' hR
+  let f (x : { x : α // x = a' ∨ TransGen R x a' }) : { x : α // x = a ∨ TransGen R x a } :=
+    ⟨x.val, by
+      cases x.property
+      · rename_i h
+        rw [h]
+        exact Or.inr (.single hR)
+      · rename_i h
+        apply Or.inr
+        refine TransGen.trans h ?_
+        exact .single hR⟩
+  have := extrinsicFix_invImage (C := (C ·.val)) (R := (R ·.1 ·.1)) (f := f) (F := fun a r => F a.1 fun a' hR => r ⟨a', Or.inr (by cases a.2; grind [TransGen.single]; exact .trans (.single hR) ‹_›)⟩ hR)
+    (F' := fun a r => F a.1 fun a' hR => r ⟨a', by cases a.2; grind [TransGen.single]; exact Or.inr (.trans (.single hR) ‹_›)⟩ hR)
+  unfold InvImage at this
+  rw [this]
+  · grind
+  · constructor
+    intro x
+    refine InvImage.accessible _ ?_
+    cases x.2 <;> rename_i hx
+    · rwa [hx]
+    · exact h.invTransGen hx
+  · constructor
+    intro x
+    refine InvImage.accessible _ ?_
+    cases x.2 <;> rename_i hx
+    · rwa [hx]
+    · exact h.invTransGen hx
+
+@[inline]
+public def WellFounded.partialExtrinsicFix₂ [∀ a b, Nonempty (C₂ a b)]
+    (R : (a : α) ×' β a → (a : α) ×' β a → Prop)
+    (F : (a : α) → (b : β a) → ((a' : α) → (b' : β a') → R ⟨a', b'⟩ ⟨a, b⟩ → C₂ a' b') → C₂ a b)
+    (a : α) (b : β a) :
+    C₂ a b :=
+  extrinsicFix₂ (α := α) (β := fun a' => { b' : β a' // (PSigma.mk a' b') = (PSigma.mk a b) ∨ TransGen R ⟨a', b'⟩ ⟨a, b⟩ })
+      (C₂ := (C₂ · ·.1))
+      (fun p q => R ⟨p.1, p.2.1⟩ ⟨q.1, q.2.1⟩)
+      (fun a b recur => F a b.1 fun a' b' hR => recur a' ⟨b', Or.inr (by
+        cases b.property
+        · grind [TransGen.single]
+        · apply TransGen.trans ?_ ‹_›
+          apply TransGen.single
+          assumption)⟩ ‹_›) a ⟨b, Or.inl rfl⟩
+
+public theorem WellFounded.partialExtrinsicFix₂_eq_partialExtrinsicFix [∀ a b, Nonempty (C₂ a b)]
+    (R : (a : α) ×' β a → (a : α) ×' β a → Prop)
+    (F : (a : α) → (b : β a) → ((a' : α) → (b' : β a') → R ⟨a', b'⟩ ⟨a, b⟩ → C₂ a' b') → C₂ a b)
+    (a : α) (b : β a) (h : Acc R ⟨a, b⟩) :
+    partialExtrinsicFix₂ R F a b = partialExtrinsicFix (α := PSigma β) (C := fun a => C₂ a.1 a.2) R (fun p r => F p.1 p.2 fun a' b' hR => r ⟨a', b'⟩ hR) ⟨a, b⟩ := by
+  simp only [partialExtrinsicFix, partialExtrinsicFix₂, extrinsicFix₂]
+  let f (x : ((a' : α) ×' { b' // PSigma.mk a' b' = ⟨a, b⟩ ∨ TransGen R ⟨a', b'⟩ ⟨a, b⟩ })) : { a' // a' = ⟨a, b⟩ ∨ TransGen R a' ⟨a, b⟩ } :=
+    ⟨⟨x.1, x.2.1⟩, x.2.2⟩
+  have := extrinsicFix_invImage (C := fun a => C₂ a.1.1 a.1.2) (f := f) (R := (R ·.1 ·.1))
+    (F := fun a r => F a.1.1 a.1.2 fun a' b' hR => r ⟨⟨a', b'⟩, ?refine_a⟩ hR)
+    (F' := fun a r => F a.1 a.2.1 fun a' b' hR => r ⟨a', b', ?refine_b⟩ hR)
+    (a := ⟨a, b, ?refine_c⟩); rotate_left
+  · cases a.2 <;> rename_i heq
+    · rw [heq] at hR
+      exact .inr (.single hR)
+    · exact .inr (.trans (.single hR) heq)
+  · cases a.2.2 <;> rename_i heq
+    · rw [heq] at hR
+      exact .inr (.single hR)
+    · exact .inr (.trans (.single hR) heq)
+  · exact .inl rfl
+  unfold InvImage f at this
+  simp at this
+  rw [this]
+  constructor
+  intro x
+  apply InvImage.accessible
+  cases x.2 <;> rename_i heq
+  · rwa [heq]
+  · exact h.invTransGen heq
+
+public def WellFounded.partialExtrinsicFix₂_eq [∀ a b, Nonempty (C₂ a b)]
+    {R : (a : α) ×' β a → (a : α) ×' β a → Prop}
+    {F : (a : α) → (b : β a) → ((a' : α) → (b' : β a') → R ⟨a', b'⟩ ⟨a, b⟩ → C₂ a' b') → C₂ a b}
+    {a : α} {b : β a} (h : Acc R ⟨a, b⟩) :
+    partialExtrinsicFix₂ R F a b = F a b (fun a' b' _ => partialExtrinsicFix₂ R F a' b') := by
+  simp only [partialExtrinsicFix₂_eq_partialExtrinsicFix (h := h)]
+  rw [partialExtrinsicFix_eq (h := h)]
+  have (a' b') (hR : R ⟨a', b'⟩ ⟨a, b⟩) : Acc R ⟨a', b'⟩ := h.inv hR
+  conv => rhs; congr; intro a' b' hR; rw [partialExtrinsicFix₂_eq_partialExtrinsicFix (h := this a' b' hR)]
+
+end Extrinsic
+
+/-!
+## Implementation 2: no termination proof required
+
+We now show an alternative implementation that does not require passing a termination proof
+as an argument. This makes the function easier to call, but verification is only possible
+on inputs where the Collatz sequence actually terminates.
+-/
+
+def oddCollatz₂ (n : Nat) : List Nat :=
+  (collectOddCollatz n ∅).toList
+where
+  -- This function is recursive and, depending on the Collatz conjecture, it may or may not terminate.
+  -- By relying on the fixpoint combinator `partialExtrinsicFix₂` instead of using the `partial` modifier,
+  -- we will be able to verify the function whenever the Collatz sequence terminates after
+  -- finitely many steps. A termination proof is not required for *calling* this function,
+  -- only for verifying it.
+  collectOddCollatz : (n : Nat) → (acc : TreeSet Nat compare) → TreeSet Nat compare :=
+    -- `partialExtrinsicFix₂` is a fixpoint combinator that produces a function that may or may
+    -- not terminate. It can be verified on inputs on which the fixpoint is well-founded.
+    -- If we had used the `partial` modifier instead, no verification would be possible at all.
+    WellFounded.partialExtrinsicFix₂ (CollatzRel ·.1 ·.1) fun n acc recur =>
+      if h : n > 1 then
+        recur (collatzStep n) (if n % 2 = 0 then acc else acc.insert n) (by grind [CollatzRel])
+      else if n = 1 then
+        acc.insert 1
+      else
+        acc
+
+/-!
+## Tests for `oddCollatz₂`
+-/
+
+example : oddCollatz₂ 14 = [1, 5, 7, 11, 13, 17] := by native_decide
+example : oddCollatz₂ 5 = [1, 5] := by native_decide
+example : oddCollatz₂ 12 = [1, 3, 5] := by native_decide
+example : oddCollatz₂ 1 = [1] := by native_decide
+
+/-!
+## Verification of `oddCollatz₂`
+
+We'll verify `oddCollatz₂` by proving it equivalent to `oddCollatz₁`.
+-/
+
+theorem collectOddCollatz_eq_collectOddCollatz {m} (hm : Acc CollatzRel m) :
+    oddCollatz₂.collectOddCollatz m acc = oddCollatz₁.collectOddCollatz ⟨m, hm⟩ acc := by
+  rw [oddCollatz₂.collectOddCollatz]
+  induction hm generalizing acc
+  rename_i h ih
+  rw [WellFounded.partialExtrinsicFix₂_eq, oddCollatz₁.collectOddCollatz]
+  · congr; ext h
+    apply ih
+    exact collatzRel_collatzStep h
+  · change Acc (InvImage CollatzRel PSigma.fst) _
+    refine InvImage.accessible _ ?_
+    exact ⟨_, h⟩
+
+theorem oddCollatz₂_eq_oddCollatz₁ {n : Nat} (hn : Acc CollatzRel n) :
+    oddCollatz₂ n = oddCollatz₁ n hn := by
+  rw [oddCollatz₂, oddCollatz₁]
+  grind [collectOddCollatz_eq_collectOddCollatz]
+
+theorem mem_oddCollatz₂_iff {m n : Nat} (h : Acc CollatzRel n) :
+    m ∈ oddCollatz₂ n ↔ m % 2 = 1 ∧ (m = n ∨ Relation.TransGen CollatzRel m n) := by
+  grind [mem_oddCollatz₁_iff, oddCollatz₂_eq_oddCollatz₁]
 
 /-!
 ## Prompt
@@ -11,12 +436,12 @@ def get_odd_collatz(n):
     Given a positive integer n, return a sorted list that has the odd numbers in collatz sequence.
 
     The Collatz conjecture is a conjecture in mathematics that concerns a sequence defined
-    as follows: start with any positive integer n. Then each term is obtained from the 
-    previous term as follows: if the previous term is even, the next term is one half of 
+    as follows: start with any positive integer n. Then each term is obtained from the
+    previous term as follows: if the previous term is even, the next term is one half of
     the previous term. If the previous term is odd, the next term is 3 times the previous
     term plus 1. The conjecture is that no matter what value of n, the sequence will always reach 1.
 
-    Note: 
+    Note:
         1. Collatz(1) is [1].
         2. returned list sorted in increasing order.
 
@@ -29,7 +454,7 @@ def get_odd_collatz(n):
 
 ```python3
     if n%2==0:
-        odd_collatz = [] 
+        odd_collatz = []
     else:
         odd_collatz = [n]
     while n > 1:
@@ -37,7 +462,7 @@ def get_odd_collatz(n):
             n = n/2
         else:
             n = n*3 + 1
-            
+
         if n%2 == 1:
             odd_collatz.append(int(n))
 
@@ -58,4 +483,4 @@ def check(candidate):
     assert candidate(1) == [1], "This prints if this assert fails 2 (also good for debugging!)"
 
 ```
--/
\ No newline at end of file
+-/