diff --git a/src/SplayHeap.agda b/src/SplayHeap.agda
index 3536350..964fcd2 100644
--- a/src/SplayHeap.agda
+++ b/src/SplayHeap.agda
@@ -24,9 +24,10 @@ open import Relation.Binary.PropositionalEquality using (_≡_; refl; cong; modu
 open import Relation.Nullary.Construct.Add.Extrema
 open import Relation.Nullary.Decidable.Core
 
-data Tree (l u : Carrier ±) : Set (c ⊔ ℓ₂) where
-  leaf : .(l ≤± u) → Tree l u
-  node : (x : Carrier) → Tree l [ x ] → Tree [ x ] u → Tree l u
+private
+  data Tree (l u : Carrier ±) : Set (c ⊔ ℓ₂) where
+    leaf : .(l ≤± u) → Tree l u
+    node : (x : Carrier) → Tree l [ x ] → Tree [ x ] u → Tree l u
 
 Heap : Set (c ⊔ ℓ₂)
 Heap = Tree ⊥± ⊤±