For those experts that can't stop after AoC2019
I have posted some rather old code in traditional style to improve it.

It was written based on a previous trial in .XLS
It is far from being perfect. Rather a challenge for
improvement in all directions (code, interface, ...)
So anyone may feel invited to make it better.

USER>d ^SUDOKU
;; Welcome to this SUDOKU demo !
;; you may solve your SUDOKU easier or create your own
;; under each box entry you find a list
;; of allowed values for this box
;; fill in numbers 1..9 as you need
;; to clear a box enter any character or blank
;; to stop enter . or , for the solver enter ?
;; navigate between boxes by cursor keys <>^v
;;
go:

This is a coding example working on Caché 2018.1.3 and IRIS 2020.2 
It will not be kept in sync with new versions 
It is also NOT serviced by InterSystems Support !

download here