code: protected function keyForScope($node) {
      return implode('-', array_map(function($column) use ($node) {
        $value = $node->getAttribute($column);
  
        if ( is_null($value) )
          return 'NULL';
  
        return $value;
      }, $node->getScopedColumns()));
    }
docs: Builds a single string for the given scope columns values. Useful for
  making array keys for grouping.
  
  @param Baum\Node   $node
  @return string