Rename 'kohana' to 'system' to conform to the Kohana filesystem layout. I'm comfortable with us not clearly drawing the distinction about the fact that it's Kohana.

system/libraries/Database_Expression.php

@@ -0,0 +1,26 @@
+<?php defined('SYSPATH') OR die('No direct access allowed.');
+/**
+ * Database expression class to allow for explicit joins and where expressions.
+ *
+ * $Id: Database_Expression.php 4037 2009-03-04 23:35:53Z jheathco $
+ *
+ * @package    Core
+ * @author     Kohana Team
+ * @copyright  (c) 2007-2009 Kohana Team
+ * @license    http://kohanaphp.com/license.html
+ */
+class Database_Expression_Core {
+
+	protected $expression;
+
+	public function __construct($expression)
+	{
+		$this->expression = $expression;
+	}
+
+	public function __toString()
+	{
+		return (string) $this->expression;
+	}
+
+} // End Database Expr Class