Lisp is fun!

I wrote a toy Lisp interpreter this weekend. Programming is pretty much a video game!

https://bitbucket.org/dhoskin/thp

(LET ((f (LAMBDA (x) (IF (x (f (CDR x))) recursion!))))
(f (QUOTE (a b c d e))))

→ args: ((a b c d e))
→ args: ((b c d e))
→ args: ((c d e))
→ args: ((d e))
→ args: ((e))
→ args: (nil)
→ recursion!

>>5

Why is f being defined twice in the LET?
First f is set to a recursive definition (where the if clause doesn't seem to have an else case),
Then g is set to f,
Then f is set to a simple function.

What is that thing supposed to do?