`pipel` and `piper` do the same thing

[DavidDTA]

Proof of concept: https://ellie-app.com/4bM4dYd2VNYa1/1

I would expect that using pipel would append "12" on each click and piper would append "21". However both versions append "21".

If this is not a bug, it warrants documentation. If it is, it is a one-character fix (change List.foldl to List.foldr in the definition of pipel, although actually I think it's better to use List.foldl for both and switch the operators for a whopping total of 5 characters).

[Fresheyeball]

Feel free to PR.