diff --git a/.php-cs-fixer.dist.php b/.php-cs-fixer.dist.php
index ed3807f..2f929af 100644
--- a/.php-cs-fixer.dist.php
+++ b/.php-cs-fixer.dist.php
@@ -12,5 +12,6 @@
     'phpdoc_align' => ['align' => 'left'],
     'phpdoc_to_comment' => false,
     'concat_space' => ['spacing' => 'one'],
-    'static_lambda' => true
+    'static_lambda' => true,
+    'single_line_empty_body' => false
 ])->setFinder($finder);
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 34039c5..7a7304c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic versioning](http://semver.org/).
 
+## [1.4.0] - 2023-09-27
+
+- introduce `OppositeSemigroup` and `OppositMonoid`
+- do not flip arguments on `ListL/ConcatenationMonoid`
+
 ## [1.3.0] - 2023-08-23
 
 - introduce `Alt` and `Plus` typeclasses
diff --git a/bin/php-cs-fixer b/bin/php-cs-fixer
index c587315..d4d7eab 100755
--- a/bin/php-cs-fixer
+++ b/bin/php-cs-fixer
@@ -1,3 +1,3 @@
 #!/usr/bin/env bash
 
-docker run --rm -v $(pwd):/app -w=/app php:8.1.0 vendor/bin/php-cs-fixer fix src --allow-risky=yes --format=checkstyle "$@"
+docker run --rm -v $(pwd):/app -w=/app php:8.1.0 vendor/bin/php-cs-fixer fix src -vvv --allow-risky=yes --format=checkstyle "$@"
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);
     }
 }
diff --git a/src/Instances/OppositeMonoid.php b/src/Instances/OppositeMonoid.php
new file mode 100644
index 0000000..d2e924f
--- /dev/null
+++ b/src/Instances/OppositeMonoid.php
@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Marcosh\LamPHPda\Instances;
+
+use Marcosh\LamPHPda\Typeclass\Monoid;
+
+/**
+ * @template A
+ *
+ * @implements Monoid<A>
+ *
+ * @psalm-immutable
+ */
+final class OppositeMonoid implements Monoid
+{
+    /** @var Monoid<A> */
+    private Monoid $monoid;
+
+    /**
+     * @param Monoid<A> $monoid
+     */
+    public function __construct(Monoid $monoid)
+    {
+        $this->monoid = $monoid;
+    }
+
+    /**
+     * @return A
+     */
+    public function mempty()
+    {
+        return $this->monoid->mempty();
+    }
+
+    /**
+     * @param A $a
+     * @param A $b
+     * @return A
+     */
+    public function append($a, $b)
+    {
+        return (new OppositeSemigroup($this->monoid))->append($a, $b);
+    }
+}
diff --git a/src/Instances/OppositeSemigroup.php b/src/Instances/OppositeSemigroup.php
new file mode 100644
index 0000000..e35499c
--- /dev/null
+++ b/src/Instances/OppositeSemigroup.php
@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Marcosh\LamPHPda\Instances;
+
+use Marcosh\LamPHPda\Typeclass\Semigroup;
+
+/**
+ * @template A
+ *
+ * @implements Semigroup<A>
+ *
+ * @psalm-immutable
+ */
+final class OppositeSemigroup implements Semigroup
+{
+    /** @var Semigroup<A> */
+    private Semigroup $semigroup;
+
+    /**
+     * @param Semigroup<A> $semigroup
+     */
+    public function __construct(Semigroup $semigroup)
+    {
+        $this->semigroup = $semigroup;
+    }
+
+    /**
+     * @param A $a
+     * @param A $b
+     * @return A
+     */
+    public function append($a, $b)
+    {
+        return $this->semigroup->append($b, $a);
+    }
+}