Ignores elements from the upstream publisher until it receives an element from a second publisher.
- iOS 13.0+
- macOS 10.15+
- Mac Catalyst 13.0+
- tvOS 13.0+
- watchOS 6.0+
- Xcode 11.0+
A publisher to monitor for its first emitted element.
A publisher that drops elements from the upstream publisher until the
other publisher produces a value.