diff --git a/spec/OppositeMonoidSpec.php b/spec/OppositeMonoidSpec.php
new file mode 100644
index 0000000..da3ef74
--- /dev/null
+++ b/spec/OppositeMonoidSpec.php
@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+use Marcosh\LamPHPda\Instances\LinkedList\LinkedListMonoid;
+use Marcosh\LamPHPda\Instances\OppositeMonoid;
+use Marcosh\LamPHPda\Instances\OppositeSemigroup;
+use Marcosh\LamPHPda\LinkedList;
+
+describe('ListTOppositeMonoid', function () {
+    it('has the same empty element', function () {
+        expect((new OppositeMonoid(new LinkedListMonoid()))->mempty())->toEqual((new LinkedListMonoid())->mempty());
+    });
+});
+
+describe('ListTOppositeSemigroup', function () {
+    it('reverses the result of append', function () {
+        expect((new OppositeSemigroup(new LinkedListMonoid()))->append(LinkedList::fromList([1]), LinkedList::fromList([2])))
+            ->toEqual((new LinkedListMonoid())->append(LinkedList::fromList([2]), LinkedList::fromList([1])));
+    });
+});
\ No newline at end of file
diff --git a/src/Instances/OppositeMonoid.php b/src/Instances/OppositeMonoid.php
index b4d9a64..16f1922 100644
--- a/src/Instances/OppositeMonoid.php
+++ b/src/Instances/OppositeMonoid.php
@@ -25,7 +25,7 @@ public function __construct(private readonly Monoid $monoid)
     /**
      * @return A
      */
-    public function mempty()
+    public function mempty(): mixed
     {
         return $this->monoid->mempty();
     }
@@ -35,7 +35,7 @@ public function mempty()
      * @param A $b
      * @return A
      */
-    public function append($a, $b)
+    public function append($a, $b): mixed
     {
         return (new OppositeSemigroup($this->monoid))->append($a, $b);
     }
diff --git a/src/Instances/OppositeSemigroup.php b/src/Instances/OppositeSemigroup.php
index 7fc324a..56e5080 100644
--- a/src/Instances/OppositeSemigroup.php
+++ b/src/Instances/OppositeSemigroup.php
@@ -27,7 +27,7 @@ public function __construct(private readonly Semigroup $semigroup)
      * @param A $b
      * @return A
      */
-    public function append($a, $b)
+    public function append($a, $b): mixed
     {
         return $this->semigroup->append($b, $a);
     }