diff --git a/src/Instances/ListL/ConcatenationMonoid.php b/src/Instances/ListL/ConcatenationMonoid.php
index 6122282..8187c53 100644
--- a/src/Instances/ListL/ConcatenationMonoid.php
+++ b/src/Instances/ListL/ConcatenationMonoid.php
@@ -29,6 +29,6 @@ public function mempty(): array
      */
     public function append($a, $b)
     {
-        return array_merge($b, $a);
+        return array_merge($a, $b);
     }
 }