clean processed list on postprocessed_element

I think you need to clean this processed list or you may lose part of the tree. 
Example:

                                                root
                                  -------------|-----------
                                 A              B             C
                               /    \           /   \          /   \
                             X     Y        X    Y       X    Y
                                               |                     |
                                              Z                    W

If you process first A when you try to process B and C you will loose Z and W.