Provides a work chain, based on concurrency, for operator concurrencyFork.
Optional
Invoked for asynchronous iterables only, to return a chain of operators for asynchronous processing.
When nothing or null is returned, it redirects to the source iterable.
null
Invoked for synchronous iterables only, to return a chain of operators for synchronous processing.
Provides a work chain, based on concurrency, for operator concurrencyFork.