~jshholland/trybqn

use new ⋈ builtin
2013: save a character in problem 6
save some brackets in 2021 prob 7 solution
Minor improvements to 2014 solutions
Add a couple of minor cosmetic improvements
finish up last of the 2021 problems
add more solutions for 2021
start 2021 problems
save a whole character in 2017.prob6!
use •Repr instead of •Fmt for formatting numbers
more work on 2020 problems
solve one and a half more problems from 2020
Add more solutions for 2020
start solutions for 2020
add remaining solutions for 2019
Create util.bqn and import it where used

Also shorten somewhat the definition of `letters`.
start 2019 problems
fix errors in 2018 problems 8 and 9
finish off 2018 and do a little golfing
add some solutions for 2018
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