Exercise 3.3.3, subsequences

```
In a string of length n, how many of the following are there?

1. Prefixes.
2. Suffixes.
3. Proper prefixes.
4. ! Substrings.
5. ! Subsequences.
```

The current solution for the last part of the question is `Σ(i=0,n) C(n, i)`, but it can be simplified as `2^n` since each position can be either deleted or not.