Instead of the previous parenting model where parents would set
child callbacks during adoption by probing for callback setters,
child elements will instead probe their parents for notify methods
listed in the standard parent interfaces. This means that an
element cannot be half-parented to something, nor can it be
parented to two things at once. Parent elements may themselves
fulfill these interfaces, or they can pass a hook that fulfills
them to the child.
Sasha Koshka