Thanks for writing the new flatten-tree function and cleaning up 
existing duplicate functions. I tweaked its performance a bit and 
cleaned up some more duplicates. While doing that I found that I was 
confused by the existence of the just-added alias 'flatten-list', and I 
propose removing it as per the attached patch. I think people who want 
to find and use the new 'flatten-tree' function will typically be better 
served if it has just one name that fits in with existing Emacs Lisp 
functions like 'copy-tree', as opposed to also having a name that is a 
misnomer.