the currently active stream coming from an incoming element once the end of this one is reached it is swapped out by another one from the next element
returns the next element in the stream difference to next is, that the internal data position is not changed, so next still will deliver the next item from the current data position. Look ahead is mostly needed internally by possible endless data constructs which have no fixed data boundary, or index positions. (aka infinite sets, or flatmapped constructs)
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Same for flatmap to deal with element -> stream mappings