rename Mystery

just unknown, mystery should be more difficult to find out.
After all we keep run time info, so just need unknown at compile time

lib/virtual/slots/new_message_slot.rb

@@ -1,24 +1,24 @@
 module Virtual
   class NewMessageSlot < Slot
-    def initialize index , type = Mystery, value = nil
+    def initialize index , type = Unknown, value = nil
       super(index , type , value )
     end
   end
 
   class NewReturn < NewMessageSlot
-    def initialize type = Mystery, value = nil
+    def initialize type = Unknown, value = nil
       super( MESSAGE_RETURN_VALUE, type , value  )
     end
   end
 
   class NewSelf < NewMessageSlot
-    def initialize type = Mystery, value = nil
+    def initialize type = Unknown, value = nil
       super( MESSAGE_SELF , type , value  )
     end
   end
 
   class NewName < NewMessageSlot
-    def initialize type = Mystery, value = nil
+    def initialize type = Unknown, value = nil
       super( MESSAGE_NAME, type , value )
     end
   end