How To Find Out about ACL2 Functions

Most ACL2 primitives are documented. Here is the definition of app again, with the documented topics highlighted. All of the links below lead into the ACL2 online documentation, 1.5 megabytes of hyperlinked text. So follow these links around, but remember to come back here!

(defun app (x y)
  (cond ((endp x) y)
        (t (cons (car x)
                 (app (cdr x) y)))))

By following the link on endp we see that it is a Common Lisp function and is defined to be the same as atom , which recognizes non-conses. But endp has a guard. Since we are ignorning guards for now, we'll ignore the guard issue on endp.

So this definition reads ``to app x and y: if x is an atom, return y; otherwise, app (cdr x) and y and then cons (car x) onto that.''