presto-main/src/main/java/io/prestosql/sql/planner/plan/Patterns.java/Patterns/sources
class Patterns:    public static Property<PlanNode, Lookup, List<PlanNode>> sources()
    {
        return property(
                "sources",
                (PlanNode node, Lookup lookup) -> node.getSources().stream()
                        .map(source -> lookup.resolve(source))
                        .collect(toImmutableList()));
    }