API breakage:

Rename _pop method into _pop_at method
as the provided function is a _pop_at method, not a _pop method.
Remove _pop from the oplist as it doesn't follow _pop convention.

doc/API.txt

@@ -6,4 +6,5 @@ V0.1.*: * First formal version of API
 V0.2.*: * worker_sync takes a 'worker_t' as an additional parameter
 	(This allows to perform a wait-on-event instead of a busy waiting). 
 	* worker_init takes an int as numWorker instead of an unsingned int.
-	
+	* RBTREE pop method has been renamed into _pop_at
+