diff --git a/Ruler.php b/Ruler.php
index b66eeed..ca15ce9 100644
--- a/Ruler.php
+++ b/Ruler.php
@@ -139,8 +139,12 @@ public function assert ( $rule, Context $context = null ) {
      * @throw   \Hoa\Ruler\Exception
      */
     public static function interprete ( $rule ) {
-
-        return static::getInterpreter()->visit(
+        
+        $interpreter = static::getInterpreter();
+        //clear context keys
+        $interpreter->clearContextKeys();
+        
+        return $interpreter->visit(
             static::getCompiler()->parse($rule)
         );
     }
@@ -225,6 +229,17 @@ public static function getCompiler ( ) {
 
         return static::$_compiler;
     }
+    
+    /**
+     * Get context keys.
+     *
+     * @access  public
+     * @return  array
+     */
+    public function getContextKeys ( ) {
+
+        return static::getInterpreter()->getContextKeys();
+    }
 }
 
 }
diff --git a/Visitor/Interpreter.php b/Visitor/Interpreter.php
index abd1ab5..ba8d8d7 100644
--- a/Visitor/Interpreter.php
+++ b/Visitor/Interpreter.php
@@ -84,7 +84,13 @@ class Interpreter implements \Hoa\Visitor\Visit {
      */
     protected $_current = null;
 
-
+    /**
+     * Context keys.
+     *
+     * @var array
+     */
+    protected $_contextKeys = array();
+    
 
     /**
      * Visit an element.
@@ -219,9 +225,11 @@ public function visit ( \Hoa\Visitor\Element $element, &$handle = null, $eldnah
                 switch($token) {
 
                     case 'identifier':
-                        return true === $variable
-                                   ? $this->_root->variable($value)
-                                   : $value;
+                        if (true === $variable) {
+                            array_push($this->_contextKeys, $value);
+                            return $this->_root->variable($value);
+                        }
+                        return $value;
 
                     case 'true':
                         return true;
@@ -269,6 +277,27 @@ public function getRoot ( ) {
 
         return $this->_root;
     }
+    
+    /**
+     * Get context keys.
+     *
+     * @access  public
+     * @return  array
+     */
+    public function getContextKeys ( ) {
+
+        return $this->_contextKeys;
+    }
+    
+    /**
+     * Clear context keys.
+     *
+     * @access  public
+     */
+    public function clearContextKeys ( ) {
+        
+        $this->_contextKeys = array();
+    }
 }
 
 }