presto-main/src/main/java/io/prestosql/sql/planner/sanity/ValidateAggregationsWithDefaultValues.java/Visitor/aggregatedSeenExchanges
class Visitor:        private Optional<SeenExchanges> aggregatedSeenExchanges(List<PlanNode> nodes)
        {
            return nodes.stream()
                    .map(source -> source.accept(this, null))
                    .reduce((accumulatorOptional, seenExchangesOptional) -> combine(accumulatorOptional, seenExchangesOptional,
                            (accumulator, seenExchanges) -> new SeenExchanges(
                                    accumulator.localRepartitionExchange && seenExchanges.localRepartitionExchange,
                                    accumulator.remoteRepartitionExchange && seenExchanges.remoteRepartitionExchange)))
                    .orElse(Optional.empty());
        }