diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e4a3818..a0055f9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,7 +7,7 @@ on:
 name: "CI"
 
 env:
-  COMPOSER_ROOT_VERSION: "3.0-dev"
+  COMPOSER_ROOT_VERSION: "3.1-dev"
 
 permissions:
   contents: read
diff --git a/ChangeLog.md b/ChangeLog.md
index 8504fce..0d8823f 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -2,6 +2,12 @@
 
 All notable changes are documented in this file using the [Keep a CHANGELOG](https://keepachangelog.com/) principles.
 
+## [3.1.0] - 2023-MM-DD
+
+### Added
+
+* `ComplexityCollection::mergeWith()`
+
 ## [3.0.1] - 2023-08-31
 
 ### Fixed
@@ -36,6 +42,7 @@ All notable changes are documented in this file using the [Keep a CHANGELOG](htt
 
 * Initial release
 
+[3.1.0]: https://github.com/sebastianbergmann/complexity/compare/3.0.1...main
 [3.0.1]: https://github.com/sebastianbergmann/complexity/compare/3.0.0...3.0.1
 [3.0.0]: https://github.com/sebastianbergmann/complexity/compare/2.0.2...3.0.0
 [2.0.2]: https://github.com/sebastianbergmann/complexity/compare/2.0.1...2.0.2
diff --git a/composer.json b/composer.json
index f3663cb..78f6ffb 100644
--- a/composer.json
+++ b/composer.json
@@ -37,7 +37,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-main": "3.0-dev"
+            "dev-main": "3.1-dev"
         }
     }
 }
diff --git a/src/Complexity/ComplexityCollection.php b/src/Complexity/ComplexityCollection.php
index 6000973..b17616c 100644
--- a/src/Complexity/ComplexityCollection.php
+++ b/src/Complexity/ComplexityCollection.php
@@ -9,6 +9,7 @@
  */
 namespace SebastianBergmann\Complexity;
 
+use function array_merge;
 use function count;
 use Countable;
 use IteratorAggregate;
@@ -75,4 +76,14 @@ public function cyclomaticComplexity(): int
 
         return $cyclomaticComplexity;
     }
+
+    public function mergeWith(self $other): self
+    {
+        return new self(
+            array_merge(
+                $this->asArray(),
+                $other->asArray(),
+            ),
+        );
+    }
 }
diff --git a/tests/unit/ComplexityCollectionTest.php b/tests/unit/ComplexityCollectionTest.php
index 29c2cfd..e209006 100644
--- a/tests/unit/ComplexityCollectionTest.php
+++ b/tests/unit/ComplexityCollectionTest.php
@@ -73,4 +73,20 @@ public function testHasCyclomaticComplexity(): void
 
         $this->assertSame(3, $collection->cyclomaticComplexity());
     }
+
+    public function testCanBeMerged(): void
+    {
+        $a = ComplexityCollection::fromList($this->array[0]);
+        $b = ComplexityCollection::fromList($this->array[1]);
+
+        $c = $a->mergeWith($b);
+
+        $this->assertSame(
+            [
+                $this->array[0],
+                $this->array[1],
+            ],
+            $c->asArray(),
+        );
+    }
 }