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# Building Hierarchy

## Composing pairs and lists

**Test Your Understanding**

Suppose we evaluate the expression
## Takeaways

### Before We Continue...

Previously we saw how to use `cons`

to "group" a pair of values together,
e.g. `(cons 1 2)`

, which returns a pair `(1 . 2)`

. We can also use `list`

to
group an arbitary amount of data together. For example if you type ```
(list 1 2
'bagel 4)
```

in the interpreter, Racket will print the list `(1 2 bagel 4)`

.
Notice that we can put *any* sort of data inside them, even other pairs and lists!

Now let's make a pair of lists:

```
(cons (list 1 2) (list 3 4))
```

The first item of the pair is the list `(1 2)`

and the second is the list `(3 4)`

. We can show this structure with the following box-and-pointer diagram:

(If you aren't familiar with drawing and interpreting box-and-pointer diagrams, please go back and review the section in Lesson 4.)

You can also represent the structure `((1 2) 3 4)`

using a **little-t tree**:

With litle-t trees, every element in a sequence is a node. In the example above, `(1 2)`

is an element of `((1 2) 3 4)`

,
so it's a node. But it's also a tree with two children nodes—one for each element.

Why do we call this a "little-t tree"? Later on in this lesson, we'll discuss the "capital-T Tree" data type, which is *completely
different* from the little-t tree data type. We use this notation for the sake of consistency and clarity.

We may also refer to little-t trees as **deep lists** (since they are lists within lists within lists within...), which is less ambiguous, but also less descriptive of the tree-like structure of lists of lists.

Suppose we evaluate the expression

`(list 1 (list 2 (list 3 4)))`

. What is returned when we enter this into the interpreter? Draw for yourself the corresponding box-and-pointer structure and the corresponding little-t tree.
In this section, we discussed nested `cons`

structures. We also introduced little-t trees.

Review the shorthand notation for `car`

s and `cdr`

s.
It will come in handy!