This would previously break because begin/end functions for each tree type still have some checks of the ntree->execdata pointer in them, despite the intended use of execdata instances instead of trees themselves for execution data storage. This is an artifact of the old execution system that required these checks to be made in the functions to avoid multiple execution of top-level trees. Now these functions take an additional argument, so group nodes can prevent them from setting and checking the nodetree->execdata pointers.
This branch adds mostly organizational improvements to the node system by renaming the node folders and files. A couple of internal features have been added too.
Detailed information can be found on the wiki page:
http://wiki.blender.org/index.php/User:Phonybone/Particles2010
