Pretty much anything. I mean, even consider a text editor, monospace font, no syntax highlighting. It's already pretty daunting. Doable, certainly, but a big task. We're lucky people already made some, and we can copy their designs even when we choose not to copy their exact code.

Now consider Microsoft Word.

Are you nuts?

Excel is great, but it's an arcane beast rivaled only by Emacs configs. People pass down Excel formulas by word of mouth like they're magic spells.