code: protected function scopedKey($node) {
      $attributes = $this->scopedAttributes($node);
  
      $output = array();
  
      foreach($attributes as $fld => $value)
        $output[] = $this->qualify($fld).'='.(is_null($value) ? 'NULL' : $value);
  
      // NOTE: Maybe an md5 or something would be better. Should be unique though.
      return implode(',', $output);
    }
docs: Return a string-key for the current scoped attributes. Used for index
  computing when a scope is defined (acsts as an scope identifier).
  
  @param   Baum\Node $node
  @return  string