presto-main/src/main/java/io/prestosql/execution/scheduler/PhasedExecutionSchedule.java/Visitor/processFragment
class Visitor:        public Set<PlanFragmentId> processFragment(PlanFragmentId planFragmentId)
        {
            if (fragmentSources.containsKey(planFragmentId)) {
                return fragmentSources.get(planFragmentId);
            }

            Set<PlanFragmentId> fragment = processFragment(fragments.get(planFragmentId));
            fragmentSources.put(planFragmentId, fragment);
            return fragment;
        }