allow overriding of attributes allocation

luatex-basics-nod.lua.patch

--- /tmp/luatex-basics-nod.lua	2013-04-16 13:56:51.387870041 +0200
+++ luatex-basics-nod.lua	2013-04-16 13:57:39.235889616 +0200
@@ -28,21 +28,23 @@
 
 end
 
-attributes            = { }
+attributes            = attributes or { }
 attributes.unsetvalue = -0x7FFFFFFF
 
 local numbers, last = { }, 127
 
-function attributes.private(name)
-    local number = numbers[name]
-    if not number then
-        if last < 255 then
-            last = last + 1
+if not attributes.private then
+    function attributes.private(name)
+        local number = numbers[name]
+        if not number then
+            if last < 255 then
+                last = last + 1
+            end
+            number = last
+            numbers[name] = number
         end
-        number = last
-        numbers[name] = number
+        return number
     end
-    return number
 end
 
 -- Nodes: