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Recap
2. Recap
Using sets to compute unique elements
Set operations
Simulating machines
Summary

## Instruction

Before we dive into the actual applications of sets, let's do a quick review.

As you may remember, sets are unordered collections of elements – there is no "first" or "last" element. Also, sets can only store unique elements; duplicates are not allowed.

Let's take a look at some basic set operations:

1. Define a set:
2. empty_set = set()
letters = {'a', 'd', 'f', 'h'}
4. letters.add('t')
5. Remove one element:
6. letters.discard('d')
7. Remove all elements:
8. letters.clear()
9. Iterate over a set:
10. for letter in letters:
...
11. Check if an element is in a set:
12. if letter in letters:
...

How about a quick warmup exercise?

## Exercise

Write a function named count_unique_letters(word) that accepts a string argument and returns the number of unique letters (characters) in the word.

### Stuck? Here's a hint!

Within a loop, add each letter of the string to a set. Then, return the length of the set with:

len(unique_letters)