Returns a token that reports cancelled if either a or b is cancelled, and whose onCancel listener fires at most once (even if both inputs later cancel).
Returns a token that reports cancelled if either a or b is cancelled, and whose onCancel listener fires at most once (even if both inputs later cancel).
Used to compose two independent cancel sources — e.g. a route-level cancel (mux-owned) with a per-call cancel (caller-supplied) — into a single token that downstream transport (AsyncByteChannel.write) can observe.