docs: public Collection<H> lowerDescendants( BitSet key ) {
          List<H> vals = new LinkedList<H>();
          int l = key.length();
          if ( l == 0 || line.isEmpty() ) {
              return new ArrayList( getSortedMembers() );
          }
          int n = line.lastKey().length();
  
          if ( l > n ) { return vals; }
  
          BitSet start = new BitSet( n );
          BitSet end = new BitSet( n );
          start.or( key );
  
          start.set( l - 1 );
          end.set( n );
  
          for ( J val : line.subMap( start, end ).values() ) {
              BitSet x = val.getBitMask();
              if ( superset( x, key ) >= 0 ) {
                  vals.add( val.getValue() );
              }
          }
  
          start.clear( l - 1 );
  
          return vals;
      }