Mod6

[LeonidPryadko]

typos fixed

[Please provide a brief description of your suggested changes here. If you are
simply providing new codes / modifying existing ones, simply describe your
changes below. Don't forget to remove this text.]

New Codes:

• XYZ code
• ...

Modified Codes:

UVW code:

• added ABC decoder
• expanded citations

PQR code:

• ...

Checklist:

I remembered to:

• Include relevant citations I could think of (with \cite{...})

• Create links to the other referenced codes (with
  \hyperref[code:...]{...})

• Update the relevant meta changelog fields with my user_id (see
  users/users_db.yml; add yourself in the PR if you aren't there already)