An <initial> element: spec 3.6's way to name a compound state's default entry point via a <transition> rather than an initial attribute.

This is a slot on its parent, Statifier.Document.State.initial_element, not a kind on Statifier.Document.State. <initial> has no id attribute at all (spec 3.6), so it fails the rule the kind set follows ("a kind is a thing with an id a transition can target"). v1 gave it a synthetic id (../statifier/lib/statifier/parser/scxml/element_builder.ex:522-525) purely to fit it into an id-bearing struct, and then had to exclude that synthetic id by hand from unique-id checking and from the compiler's id map. A slot has no id to exclude in the first place.

transitions is a list, not a single nilable Transition.t(), even though the spec requires exactly one <transition> child. Both the zero-transition case and the two-or-more case must survive lowering long enough to be named: the validator's check 4 reports a <state> with both an initial attribute and an <initial> element, and the same "hold the wrong shape long enough to report it" principle applies to <initial>'s own transition count. A %Statifier.Document.State{kind: :history} already has this shape for free - its default transition is transitions: [Transition.t()] with no special casing - so the list representation costs nothing extra to reuse here.

The <initial> transition's executable content, which spec 3.6 permits, is representable at no extra cost: Transition.content already holds it.