diff --git a/HumanEvalLean/HumanEval34.lean b/HumanEvalLean/HumanEval34.lean
index c77a9f2..23dde6b 100644
--- a/HumanEvalLean/HumanEval34.lean
+++ b/HumanEvalLean/HumanEval34.lean
@@ -1,7 +1,33 @@
 module
 
-def unique : Unit :=
-  ()
+public import Std
+open Std
+
+public section
+
+/-! ## Implementation -/
+
+def unique (xs : Array Nat) : Array Nat :=
+  (TreeSet.ofList xs.toList).toArray -- TODO: use `ofArray` as soon as there are lemmas for it
+
+/-! ## Tests -/
+
+example : unique #[5, 3, 5, 2, 3, 3, 9, 0, 123] = #[0, 2, 3, 5, 9, 123] := by native_decide
+
+/-! ## Verification -/
+
+theorem pairwise_lt_unique {xs : Array Nat} :
+    (unique xs).toList.Pairwise (· < ·) := by
+  -- ideally, we'd have `ordered_toArray`
+  simpa [unique, ← TreeSet.toArray_toList, compare_eq_lt] using TreeSet.ordered_toList (α := Nat) (cmp := compare)
+
+theorem pairwise_ne_unique {xs : Array Nat} :
+    (unique xs).toList.Pairwise ( · ≠ ·) := by
+  simpa [unique, ← TreeSet.toArray_toList, compare_ne_eq] using TreeSet.distinct_toList (α := Nat) (cmp := compare)
+
+theorem mem_unique_iff {xs : Array Nat} {x : Nat} :
+    x ∈ unique xs ↔ x ∈ xs := by
+  grind [unique]
 
 /-!
 ## Prompt
@@ -34,4 +60,4 @@ def check(candidate):
     assert candidate([5, 3, 5, 2, 3, 3, 9, 0, 123]) == [0, 2, 3, 5, 9, 123]
 
 ```
--/
\ No newline at end of file
+-/
diff --git a/lean-toolchain b/lean-toolchain
index 28cffe5..dcca6df 100644
--- a/lean-toolchain
+++ b/lean-toolchain
@@ -1 +1 @@
-leanprover/lean4:nightly-2026-02-25
+lean4