articles/lisp.md: update

articles/lisp.md

@@ -60,6 +60,23 @@ Creating lists:
 (list 20 "Lisp" '(1 2) 39)
 ```
 
+Of course, cons cells are not limited to just lists, they can be nested and represent trees. Just read any Lisp source code for proof,
+you're seeing the source encoded in a tree, directly by using S-Expressions. But here is something more immediate:
+```lisp
+;; (20 * 2 + 16) / 30 * 5
+;; (cons '* (cons (cons '/ (cons (cons (cons '+ (cons '* (cons 20 2)) 16)) 30)) 5))
+;;
+;;                       (*)
+;;                      /   \
+;;                   (/)     5
+;;                  /   \
+;;                (+)    30
+;;               /   \
+;;            (*)     16
+;;           /   \
+;;         20     2
+```
+
 ## Lisp languages in use
 - [Scheme](scheme.md): created by the hackers Guy Steele and Gerald Sussman in 1975 at the MIT.
 - [Common Lisp](common_lisp.md): Big for Lisp standards (still better and smaller than "modern" languages), is also the most used Lisp today.