Note that you may also have a Lisp form like #1=(programmable . #1#), which represents not a tree but a graph. With lispm's remarks in mind, you can see that i

Note that you may also have a Lisp form like #1=(programmable . #1#), which represents not a tree but a graph. With lispm's remarks in mind, you can see that it's not abstract, not syntax, and not a tree.