Interesting. I model `array.join(',')` or the equivalent Py `','.join(l)` in my head as ultimately (pseudo-code):

    foldLeft(tail(array), str(head(elem)), (acc: String, elem: T) -> acc + ',' + str(elem))

Which makes the array method easier to understand than the string method. Funny how I thought it was _obvious_, but you find the opposite obvious.