...

........................

....

I can use dots here without upsetting mods :)

....

@D.Hutchinson ................................................................................‌​.................................................................................‌​.................................................................................‌​.................................................................................‌​.................................................................................‌​.......................................................

# calculate side length (exact)
J×’$×3‘<LS‘

# coordinate (list) -> index

ṛ/Cr⁹Ṗạ.~+⁹+⁹S+ḷ/µ³L”.ị⁹$<?

#test for the above
J×’$×3‘<LS‘ɓ  ⁹’ŒR,þ`Ç€€ G

>.< no space left on drive...