This allows for wrapping an existing stream by re-assigning its receive
method to the allocated broadcaster's `.receive()` so as to avoid
expecting any original consumer(s) of the stream to now know about the
broadcaster; this instead mutates the stream to delegate to the new
receive call behind the scenes any time `.subscribe()` is called.
Add a `typing.Protocol` for so called "cloneable channels" until we
decide/figure out a better keying system for each subscription and
mask all undesired typing failures.
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