See: https://adventofcode.com/2015/day/5
** Part 1
*** Purpose
Given a list of lines of strings:

#+begin_example
sszojmmrrkwuftyv
isaljhemltsdzlum
fujcyucsrxgatisb
qiqqlmcgnhzparyg
#+end_example

Categorize a string as favorable if it follows these rules:

1. It contains at least three vowels (aeiou only), like aei, xazegov, or aeiouaeiouaeiou.
2. It contains at least one letter that appears twice in a row, like xx, abcdde (dd), or aabbccdd (aa, bb, cc, or dd).

But does NOT:
1. Contain the strings ab, cd, pq, or xy, even if they are part of one of the other requirements.

*** Method

There are a lot of ways we can accomplish this. Personally, I prefer the obvious way:

Construct a few different predicate procedures that determine whether one of the above condictions is true.
Construct an overarching predicate function that runs all of the sub-functions, and uses and/or logic to report accept=#t/#f.

Run all strings through, returning a boolean list, and then simply count the number of '#t's.

** Part 2
*** Purpose
The next part revokes the old rules and introduces two new ones:

1. Contains a pair of any two letters that appear at least twice in the string without overlapping, like xyxy (xy) or aabcdefgaa (aa), but not like aaa (aa, but it overlaps).
2. Contains at least one letter which repeats with exactly one letter between them, like xyx, abcdefeghi (efe), or even aaa.

*** Method
The repeated-char? character must now be able to accept an argument that specifies a "gap" to look ahead.

Rather than the old way, it makes sense to now look ~x~ characters ahead on each iteration. If ~x~ characters ahead is equal to current char, then we return true.

The first rule is harder... In all cases of this "repeating char" rule, it is when the same character repeats three times.

Maybe we can store /all/ pairs of characters as an a-list, using the index of the first character as the key, and the pair as