diff --git a/mozilla/js/rhino/src/org/mozilla/javascript/UniqueTag.java b/mozilla/js/rhino/src/org/mozilla/javascript/UniqueTag.java
index 7e0c45a713f..437d0ae3b09 100644
--- a/mozilla/js/rhino/src/org/mozilla/javascript/UniqueTag.java
+++ b/mozilla/js/rhino/src/org/mozilla/javascript/UniqueTag.java
@@ -38,47 +38,78 @@ package org.mozilla.javascript;
 import java.io.Serializable;
 
 /**
-Class instances represent serializable tags to mark special values of Object references.
-<p>
-Compatibility note: under jdk 1.1 use org.mozilla.javascript.serialize.ScriptableInputStream to read serialized instances of UniqueTag as under this JDK version the default ObjectInputStream would not restore them correctly as it lacks support for readResolve method
-*/
+ * Class instances represent serializable tags to mark special Object values.
+ * <p>
+ * Compatibility note: under jdk 1.1 use
+ * org.mozilla.javascript.serialize.ScriptableInputStream to read serialized
+ * instances of UniqueTag as under this JDK version the default
+ * ObjectInputStream would not restore them correctly as it lacks support
+ * for readResolve method
+ */
 public final class UniqueTag implements Serializable
 {
-    private UniqueTag(int tagId) {
-        _tagId = tagId;
+    private static final int ID_NOT_FOUND    = 1;
+    private static final int ID_NULL_VALUE   = 2;
+    private static final int ID_DOUBLE_MARK  = 3;
+
+    /**
+     * Tag to mark non-existing values.
+     */
+    public static final UniqueTag
+        NOT_FOUND = new UniqueTag(ID_NOT_FOUND);
+
+    /**
+     * Tag to distinguish between uninitialized and null values.
+     */
+    public static final UniqueTag
+        NULL_VALUE = new UniqueTag(ID_NULL_VALUE);
+
+    /**
+     * Tag to indicate that a object represents "double" with the real value
+     * stored somewhere else.
+     */
+    public static final UniqueTag
+        DOUBLE_MARK = new UniqueTag(ID_DOUBLE_MARK);
+
+    private final int tagId;
+
+    private UniqueTag(int tagId)
+    {
+        this.tagId = tagId;
     }
 
-    public Object readResolve() {
-        if (_tagId == ID_NOT_FOUND) { return NOT_FOUND; }
-        else if (_tagId == ID_NULL_VALUE) { return NULL_VALUE; }
-        Kit.codeBug();
-        return null;
+    public Object readResolve()
+    {
+        switch (tagId) {
+          case ID_NOT_FOUND:
+            return NOT_FOUND;
+          case ID_NULL_VALUE:
+            return NULL_VALUE;
+          case ID_DOUBLE_MARK:
+            return DOUBLE_MARK;
+        }
+        throw new IllegalStateException(String.valueOf(tagId));
     }
 
 // Overridden for better debug printouts
-    public String toString() {
+    public String toString()
+    {
         String name;
-        if (_tagId == ID_NOT_FOUND) { name = "NOT_FOUND"; }
-        else if (_tagId == ID_NULL_VALUE) { name = "NULL_VALUE"; }
-        else { Kit.codeBug(); name = null; }
+        switch (tagId) {
+          case ID_NOT_FOUND:
+            name = "NOT_FOUND";
+            break;
+          case ID_NULL_VALUE:
+            name = "NULL_VALUE";
+            break;
+          case ID_DOUBLE_MARK:
+            name = "DOUBLE_MARK";
+            break;
+          default:
+            throw Kit.codeBug();
+        }
         return super.toString()+": "+name;
-
     }
 
-    private static final int ID_NOT_FOUND   = 1;
-    private static final int ID_NULL_VALUE  = 2;
-
-    private int _tagId;
-
-/**
-NOT_FOUND is useful to mark non-existing values.
-*/
-    public static final UniqueTag NOT_FOUND = new UniqueTag(ID_NOT_FOUND);
-
-/**
-NULL_VALUE is useful to distinguish between uninitialized and null values
-*/
-    public static final UniqueTag NULL_VALUE = new UniqueTag(ID_NULL_VALUE);
-
 }