code: public Set<String> keySet() {
  
          PropertyBundle p = this;
          Set<String> keys = new HashSet<>(p.key2values.keySet());
  
          while ((p = p.parent) != null) {
              keys.addAll(p.key2values.keySet());
          }
  
          return Collections.unmodifiableSet(keys);
  
      }
docs: /*[deutsch]
  <p>Ermittelt alle verkn&uuml;pften Schl&uuml;ssel. </p>
  
  @return  Set