Merge multiple sources.
Merge multiple sources. Prefer sources depending on priorities if all sources has elements ready. If a subset of all sources has elements ready the relative priorities for those sources are used to prioritise.
emits when one of the inputs has an element available, preferring inputs based on their priorities if multiple have elements available
backpressures when downstream backpressures
completes when all upstreams complete (This behavior is changeable to completing when any upstream completes by setting