fun <T, R> <ERROR CLASS><T>.fold(accumulator: R, folderFun: (R, T) -> Unit): Rfold takes an rx.Observable stream and applies fold function on it, and collects all elements using the accumulator.
accumulator - The accumulator for accumulating elements.folderFun - The transformation function to be called on the observable list when a new element is emitted on
the stream, which should modify the list as needed.