new EventPropagators()
A small set of propagation patterns, each of which will accept a small amount
of information, and generate a set of "dispatch ready event objects" - which
are sets of events that have already been annotated with a set of dispatched
listener functions/ids. The API is designed this way to discourage these
propagation strategies from actually executing the dispatches, since we
always want to collect the entire set of dispatches before executing event a
single one.
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