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results, and gathered in the result list; otherwise, the first
raised exception will be immediately propagated to the returned
future.

Cancellation: if the outer Future is cancelled, all children (that
have not completed yet) are also cancelled.  If any child is
cancelled, this is treated as if it raised CancelledError --
the outer Future is *not* cancelled in this case.  (This is to
prevent the cancellation of one child to cause other children to
be cancelled.)

If *return_exceptions* is False, cancelling gather() after it
has been marked done won't cancel any submitted awaitables.
For instance, gather can be marked done after propagating an
exception to the caller, therefore, calling ``gather.cancel()``
after catching an exception (raised by one of the awaitables) from
gather won't cancel any other awaitables.
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