Element fieldset

fieldset =>

array:9 [
  "class" => "Drupal\Core\Render\Element\Fieldset"
  "provider" => "core"
  "id" => "fieldset"
  "#process" => array:2 [
    0 => array:2 [
      0 => "Drupal\Core\Render\Element\Fieldset"
      1 => "processGroup"
    ]
    1 => array:2 [
      0 => "Drupal\Core\Render\Element\Fieldset"
      1 => "processAjaxForm"
    ]
  ]
  "#pre_render" => array:1 [
    0 => array:2 [
      0 => "Drupal\Core\Render\Element\Fieldset"
      1 => "preRenderGroup"
    ]
  ]
  "#value" => null
  "#theme_wrappers" => array:1 [
    0 => "fieldset"
  ]
  "#type" => "fieldset"
  "#defaults_loaded" => true
]