how can I handle either-or with Rx?

I need to handle an either-or type situation with the Reactive-Extensions.

I have 2 events I'm receiving (A or B). When event A comes in followed by an event B within 2 seconds I want to generate a C. If an A is not followed by B then I'd like to generate a D.

Is there some way to express that with Rx??

Thanks

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