See: https://adventofcode.com/2020/day/2
** Part 1
*** Purpose
Given a list of passwords and their supposed requirements

#+begin_example
1-3 a: abcde
1-3 b: cdefg
2-9 c: ccccccccc
#+end_example

Return the amount of valid passwords. Password one requires 1-3 'a' characters. Password 2 is invalid.

*** Method
Simply parse each input and store them in... Well, let's do a list:

#+begin_example
(passwd char (min max))
#+end_example

Then, create a function count occurances of CHAR in PASSWD, and see how it compares to RANGE.

** Part 2
*** Purpose
The password verification method has changed:

#+begin_example
1-3 a: abcde
1-3 b: cdefg
2-9 c: ccccccccc
#+end_example

Now, the "range" corresponds to two indices (indexed from 1). Of these indices, exactly one needs to be the given CHAR.

*** Method

See implementation. Simply, we add a wrapper to the valid-password? function that refines the inputs before calling valid-password? as usual.