It's pretty simple, we got a 10x10 grid, now click on any grid box and it will highlight it's symmetrical position on the other three sides. It took a lot of math, which I am very bad at.
Works perfectly only from the top-left (and bottom-left).
You might like this pen: Random Symmetrical Patterns (https://codepen.io/depy/details/EaLVdN/)
by Deepak Kamat - @xxxdepy