What is a collapsing lisp procedure, first time I have heard of this...
I am not sure what you are showing me in your example.
"Collapsing lisT" (emphasis on the T). Sometimes I have to process a *very* large list in a way that I have to find something later in the list. I could use (nth ...) to iterate over the entire list, but that can make for a very long search eg. 1st item, search against items 2 through n; 2nd item, search through items 1 and 3 through n; and so on. Or I can try to manage a secondary index of where to start searching; that gets even worse if I have to start removing that matching element from the list. Instead, on each pass I use (car ...) to get the first item in the list and (setq tlist (cdr tlist)) to set the working list as the remainder. Since the list is shrinking (collapsing) with every cycle, each pass needs to process fewer and fewer items.