Alternative combinator: try a sequence of producers in order and return the first that yields a defined value, or undefined if every one declines.

Producers run lazily and may be async (each is awaited before the next is tried), so later, more expensive lookups are skipped once one claims the input. Use it to fold an ordered set of fallible parsers/handlers into a single "first match wins" lookup instead of an if … else if … ladder. A null result counts as a match (the producer claimed the input but found it invalid), distinct from undefined ("not mine — try the next").