Good one. Do you know until what Swift version the replacingCharacters method was supported? Not anymore in 5.2, and apparently valid around 4.2.

Good one. And this can be more difficult than it looks, for anyone who has never done numerals with a programming language or a calculator.

I think the problem description requires extra information, to make it a bit clearer (the historical background is great). For example, it seems to me easier to see the target with:

• Just mentioning that Arabic numerals are 0 to 9, etc.
• Giving concrete information on what "European variety" means. "The kind used in most programming language"? Maybe a link to some definition page could be less vague.
• Perhaps an example for encode and decode.

As for the last point, I do not know what is the best practice/balance on Codewars.

Next training...