Confusing/misleading error message for a beginner

[einar]

Hi Bogumił, thanks for answering!

Yes, you are right - it is there.

Two quick documentation suggestions from a newcomer:

1. Add one extra example with this case here since what a person in my situation will do is to look at the @subset documentation. Just one more row?
   @subset(df, :category .∈ Ref(categories))

2. Add a short sentence explaining why Ref is needed (and provide link for further reading), in the place above and also in the place where you linked (in the DataFrame-docs).

You are the expert regarding this package, but I am more expert in being a newcomer than you are