Revert "remove htmlpurifier"

This reverts commit c21a462d52.

lib/htmlpurifier/library/HTMLPurifier/ChildDef/Optional.php

@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * Definition that allows a set of elements, and allows no children.
+ * @note This is a hack to reuse code from HTMLPurifier_ChildDef_Required,
+ *       really, one shouldn't inherit from the other.  Only altered behavior
+ *       is to overload a returned false with an array.  Thus, it will never
+ *       return false.
+ */
+class HTMLPurifier_ChildDef_Optional extends HTMLPurifier_ChildDef_Required
+{
+    public $allow_empty = true;
+    public $type = 'optional';
+    public function validateChildren($tokens_of_children, $config, $context) {
+        $result = parent::validateChildren($tokens_of_children, $config, $context);
+        // we assume that $tokens_of_children is not modified
+        if ($result === false) {
+            if (empty($tokens_of_children)) return true;
+            elseif ($this->whitespace) return $tokens_of_children;
+            else return array();
+        }
+        return $result;
+    }
+}
+
+// vim: et sw=4 sts=4