code: public Set<K> getUnavailableNodes() {
          final Set<K> result = new HashSet<K>();
          for ( final Map.Entry<K, ManagedItem<Boolean>> entry : _map.entrySet() ) {
              if ( !entry.getValue()._value.booleanValue() && !isExpired( entry.getValue() ) ) {
                  result.add( entry.getKey() );
              }
          }
          return result;
      }
docs: A set of nodes that are stored as unavailable.
  
  @return a set of unavailable nodes, never <code>null</code>.