src/Pure/Concurrent/mailbox.scala

--- a/src/Pure/Concurrent/mailbox.scala	Fri Nov 06 14:43:05 2015 +0100
+++ b/src/Pure/Concurrent/mailbox.scala	Fri Nov 06 18:15:35 2015 +0100
@@ -17,7 +17,7 @@
 
 class Mailbox[A] private()
 {
-  private val mailbox = Synchronized(List.empty[A])
+  private val mailbox = Synchronized[List[A]](Nil)
   override def toString: String = mailbox.value.reverse.mkString("Mailbox(", ",", ")")
 
   def send(msg: A): Unit = mailbox.change(msg :: _)