additional_fields['residues'] = [
      'table' => 'feature',
      'field' => 'residues',
    ];
  }
  /**
   * {@inheritdoc}
   */
  function query() {
    $this->ensure_my_table();
    $this->add_additional_fields();
  }
  /**
   * {@inheritdoc}
   */
  function render($values) {
    // add a 
 every 100 bp
    return ereg_replace("(.{100})", "\\1
", $values->feature_residues);
  }
}