Instructions

You! Yes you, brave lisper! Lenny the lisp alien needs your help! Lenny wants to go hang out with all the other lisp aliens, but their parents have told Lenny that they're not going anywhere until their room is clean.

The issue is, Lenny can hardly see the floor! Lenny can't tell their atoms from their conses, their symbols from their keywords, their cars from their cdrs!

Can you help Lenny sort this mess out? Don't forget your parentheses, you'll be needing them!

1. Refresh Lenny on symbols

Before Lenny can get started, they need a refresher on what symbols and keywords look like – it's been a while since they last tidied up... Help Lenny out by defining two functions: lennys-favorite-food which evaluates to some symbol, and lennys-secret-keyword which evaluates to some keyword. For example:

Note that you can return any symbol or keyword you'd like (if you're not a fan of lasagna or aren't a believer in aliens).

2. Help Lenny distinguish between atoms and conses

Once Lenny has had a refresher on symbols and keywords, it's time to get cleaning! To sort through all of the S-Expressions laying around their room, Lenny needs help determining what is a cons and what is an atom. You can help by implementing two functions, is-an-atom-p and is-a-cons-p:

For this section, there are a number of built-in lisp functions that you may find useful. Additionally, for now, you can simply consider T as true and NIL as false.

3. Help Lenny split up their conses

Finally, Lenny's conses are too bulky to put away neatly, so they need to split them up into smaller parts. Can you help by defining two more functions to break up their conses (first-thing and rest-of-it)?

Again, some of Common Lisp's built-in functions might come in handy here.