Function find 

pub fn find<F>(
    core: &Core,
    binding: &Arc<dyn OperatorBinding>,
    source: NodeId,
    predicate: F,
) -> FlowRegistration
where
    F: Fn(HandleId) -> bool + Send + Sync + 'static,

find(source, predicate) — emits the first DATA matching predicate then Complete. Sugar for take(filter(source, predicate), 1).