public Set<TracedConcept> flush() {
   if (m_Open.isEmpty()) return Collections.emptySet();
   Set<TracedConcept> openCopy = CollectionUtil.makeSet(m_Open.values());
   m_Expanded.putAll(m_Open);
   m_Open.clear();
   return openCopy;
 }
 public TracedConceptSet() {
   m_Open = CollectionUtil.makeMap();
   m_Expanded = CollectionUtil.makeMap();
 }