return Collections.unmodifiableSet(critOids);
HashSet s = new HashSet(); for (Iterator it = extensions.values().iterator(); it.hasNext(); ) { Extension e = (Extension) it.next(); if (e.isCritical()) s.add(e.getOid().toString()); } return Collections.unmodifiableSet(s);
  public Set getCriticalExtensionOIDs()  {    return Collections.unmodifiableSet(critOids);  }