These are my answers for Advent of Code 2021 in Arc. If you want to run them:

1. Create a subfolder lib with (a link to) the standard library's strings.arc and Conrad Barski's re.arc.
2. Start an Arc repl from the repository root and (load "aoc.arc")
3. Now you can use (run day part example?), with day and part being the number of the day and part to run, and example? absent or nil to use the normal input, or t to use the example input.