presto-main/src/main/java/io/prestosql/sql/planner/FragmentTableScanCounter.java/FragmentTableScanCounter/hasMultipleSources
class FragmentTableScanCounter:    public static boolean hasMultipleSources(PlanNode... nodes)
    {
        int count = 0;
        for (PlanNode node : nodes) {
            count += node.accept(new Visitor(), null);
        }
        return count > 1;
    }