presto-main/src/main/java/io/prestosql/execution/scheduler/PhasedExecutionSchedule.java/PhasedExecutionSchedule/processFragment
class PhasedExecutionSchedule:        private Set<PlanFragmentId> processFragment(PlanFragment fragment)
        {
            Set<PlanFragmentId> sources = fragment.getRoot().accept(this, fragment.getId());
            return ImmutableSet.<PlanFragmentId>builder().add(fragment.getId()).addAll(sources).build();
        }