Maybe is a mix of single and completable. It can emit a single next event, a completed event, or an error event. In other words, it can either emit an element or error and be done or emit a completed event without emitting anything else. Like single and completable, it does not replay, and we can convert a raw Observable sequence to a maybe by calling asMaybe() on it. A maybe can be used just like a single in places where it might be possible that it won't emit an element or an error and could just complete.
The maybe trait
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