presto-main/src/main/java/io/prestosql/sql/planner/plan/Patterns.java/Patterns/source
class Patterns:    public static Property<PlanNode, Lookup, PlanNode> source()
    {
        return optionalProperty(
                "source",
                (node, lookup) -> {
                    if (node.getSources().size() == 1) {
                        PlanNode source = getOnlyElement(node.getSources());
                        return Optional.of(lookup.resolve(source));
                    }
                    return Optional.empty();
                });
    }