presto-main/src/main/java/io/prestosql/execution/scheduler/AllAtOnceExecutionSchedule.java/Visitor/visitPlan
class Visitor:        @Override
        protected Void visitPlan(PlanNode node, Void context)
        {
            List<PlanNode> sources = node.getSources();
            if (sources.isEmpty()) {
                return null;
            }
            if (sources.size() == 1) {
                sources.get(0).accept(this, context);
                return null;
            }
            throw new UnsupportedOperationException("not yet implemented: " + node.getClass().getName());
        }