RxJS 1 array item into sequence of single items - operator

Question

Given such observable

Rx.Observable.of([1,2,3,4,5])

which emits a single item (that is an array), what is the operator

Answer 1

Dont understand why concatMap(x => x) or flatMap(x => x) works, it doesnt change anything. This should work (rxjs 6 or above):

of([4,5,6]).pipe(mergeMap(from))