tream</code>.</li><li>Similarly, canceling a subscription of the returned stream eventually
(in reasonable time) cancels the subscription of the input <code>stream</code>.</li></ul>
<p>"Reasonable time" depends on the transformer and stream. Some transformers,
like a "timeout" transformer, might make these operations depend on a
duration. Others might not delay them at all, or just by a microtask.</p>
<p>Transformers are free to handle errors in any way.
A transformer implementation may choose to propagate 