Thanks. I overlooked that condition.

Because of the "if i == 0" argument, the first element is just added without checking it against anything else.

Why does this work for the first element? What is ch compared with for i=0? With the last element of the sequence iterable[-1]? Tricky! But I'm not sure if in a good way or a bad way ;)

There aren't and afaik there never were nested lists in this kata.

Just want to mention that this one fails when there is a single element list inside the list like this [['a'],'a','b'].
But I think it is very likely that the full attempt has been changed in between, because this little detail is not mentioned in the description. And probably other solutions will also fail now, but this one I was interested in ;o)

Nice one-string code

First argument in call, according to documenation is "this" for the function. That allows you to write complex constructors.