> The idea was not to prefer lists (I would also prefer arrays), but to be as
> non-intrusive as possible and delay the conversion.

Right, but it makes it harder to work with the code, because we don't
know what the data type might be, and thus we have to code defensively.

> In particular here any of the inputs might be an X that is a list of strings
> and should not be converted to an array.

That's a pretty good argument. I would however say that we should convert
to arrays because if you have big data, you shouldn't be working with
strings anyhow.