diff --git a/library/HTMLPurifier/Definition.php b/library/HTMLPurifier/Definition.php
new file mode 100644
index 00000000..2a430bde
--- /dev/null
+++ b/library/HTMLPurifier/Definition.php
@@ -0,0 +1,36 @@
+<?php
+
+/**
+ * Super-class for definition datatype objects, implements serialization
+ * functions for the class.
+ */
+class HTMLPurifier_Definition
+{
+    
+    /**
+     * Has setup() been called yet?
+     */
+    var $setup = false;
+    
+    /**
+     * Sets up the definition object into the final form, something
+     * not done by the constructor
+     * @param $config HTMLPurifier_Config instance
+     */
+    function doSetup($config) {
+        trigger_error('Cannot call abstract method', E_USER_ERROR);
+    }
+    
+    /**
+     * Setup function that aborts if already setup
+     * @param $config HTMLPurifier_Config instance
+     */
+    function setup($config) {
+        if ($this->setup) return;
+        $this->setup = true;
+        $this->doSetup($config);
+    }
+    
+}
+
+?>
\ No newline at end of file
diff --git a/tests/HTMLPurifier/DefinitionTest.php b/tests/HTMLPurifier/DefinitionTest.php
new file mode 100644
index 00000000..2ce85d07
--- /dev/null
+++ b/tests/HTMLPurifier/DefinitionTest.php
@@ -0,0 +1,33 @@
+<?php
+
+require_once 'HTMLPurifier/Definition.php';
+
+Mock::generatePartial(
+        'HTMLPurifier_Definition',
+        'HTMLPurifier_Definition_Testable',
+        array('doSetup'));
+
+class HTMLPurifier_DefinitionTest extends UnitTestCase
+{
+    function test_setup() {
+        $def = new HTMLPurifier_Definition_Testable();
+        $config = HTMLPurifier_Config::createDefault();
+        $def->expectOnce('doSetup', array($config));
+        $def->setup($config);
+    }
+    function test_setup_redundant() {
+        $def = new HTMLPurifier_Definition_Testable();
+        $config = HTMLPurifier_Config::createDefault();
+        $def->expectNever('doSetup');
+        $def->setup = true;
+        $def->setup($config);
+    }
+    function test_doSetup_abstract() {
+        $def = new HTMLPurifier_Definition();
+        $this->expectError('Cannot call abstract method');
+        $config = HTMLPurifier_Config::createDefault();
+        $def->doSetup($config);
+    }
+}
+
+?>
\ No newline at end of file
diff --git a/tests/test_files.php b/tests/test_files.php
index af128882..cb184154 100644
--- a/tests/test_files.php
+++ b/tests/test_files.php
@@ -58,6 +58,7 @@ $test_files[] = 'ChildDef/TableTest.php';
 $test_files[] = 'ConfigSchemaTest.php';
 $test_files[] = 'ConfigTest.php';
 $test_files[] = 'ContextTest.php';
+$test_files[] = 'DefinitionTest.php';
 $test_files[] = 'DoctypeRegistryTest.php';
 $test_files[] = 'ElementDefTest.php';
 $test_files[] = 'EncoderTest.php';