diff --git a/src/nicoSWD/Rules/Core/CallableFunction.php b/src/nicoSWD/Rules/Core/CallableFunction.php
index fd0a8b4..40f8c4a 100644
--- a/src/nicoSWD/Rules/Core/CallableFunction.php
+++ b/src/nicoSWD/Rules/Core/CallableFunction.php
@@ -11,7 +11,7 @@
 
 use nicoSWD\Rules\Tokens\BaseToken;
 
-abstract class CallableFunction
+abstract class CallableFunction implements CallableUserFunction
 {
     /**
      * @var BaseToken
@@ -22,13 +22,4 @@ public function __construct(BaseToken $token)
     {
         $this->token = $token;
     }
-
-    /**
-     * @param BaseToken $param
-     * @param BaseToken $param...
-     * @return mixed
-     */
-    abstract public function call($param = null);
-
-    abstract public function getName() : string;
 }
diff --git a/src/nicoSWD/Rules/Core/CallableUserFunction.php b/src/nicoSWD/Rules/Core/CallableUserFunction.php
new file mode 100644
index 0000000..e7b3069
--- /dev/null
+++ b/src/nicoSWD/Rules/Core/CallableUserFunction.php
@@ -0,0 +1,22 @@
+<?php
+
+/**
+ * @license     http://opensource.org/licenses/mit-license.php MIT
+ * @link        https://github.com/nicoSWD
+ * @author      Nicolas Oelgart <nico@oelgart.com>
+ */
+namespace nicoSWD\Rules\Core;
+
+use nicoSWD\Rules\Tokens\BaseToken;
+
+interface CallableUserFunction
+{
+    /**
+     * @param BaseToken $param
+     * @param BaseToken $param ...
+     * @return BaseToken
+     */
+    public function call($param = null);
+
+    public function getName() : string;
+}