A menu link plugin ID that should be used as the root.
By default the root ID of empty string '' is used. However, when only the descendants (subtree) of a certain menu link are needed, a custom root can be specified.
Type: string
public $root = '';
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Menu!MenuTreeParameters.php/property/MenuTreeParameters::root/8.1.x