The following line is not entirely correct:
The Observable.concat(_ : ) static method takes an ordered collection of observables (i.e. an array).
This is related to the code:
let observable = Observable.concat([first, second])
It works OK without brackets, i.e.:
let observable = Observable.concat(first, second)
Even more, this one works OK too:
let first = Observable.of(1, 2, 3)
let second = Observable.of(4, 5, 6)
let three = Observable.of(77)
let observable = Observable.concat(first, second, three)
So, I guess, words i.e. an array in that line is kind of misleading.