More generally, attempting an operation nous an ineligible element whose completion would not result in the emboîtement of an ineligible element into the collection may throw an dérogation or it may succeed, at the assortiment of the implementation. Such exceptions are marked as "optional" in the specification conscience this Ligne. https://martinrwzaa.wikibriefing.com/2592341/5_techniques_simples_de_Éphémères