I thought I'd give the 99 Lisp Problems a spin as a way of exploring Pico Lisp. One of my first outings came up with the discovery that if you call a function without enough arguments the remaining arguments get set to NIL this can be used to do default arguments as follows:

(de fun (X Y)

(let (Y (or Y 5))

(+ x Y)))

(fun 1 2) -> 2

(fun 1) -> 6

I did this to simulate an accumulator argument in a recursive function. But then i got to thinking, I'm executing the let on each recursive call, which seems kind of a waste as it is only needed on the first call. Well Pico has an answer for this recur and recurse which allows you to define an anonymous recursive function. Now this is something you don't find in your garden variety common lisp!

(de my-rev (lst)

(let (res ())

(recur (lst res)

(if (pair lst)

(recurse (cdr lst) (cons (car lst) res))

res))))

That is one neat way to make do without auxiliary functions.