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/// A list with a single element
Leaf(Primitive<'gc>),
/// A pair of list cells (car, cdr)
Cons(Gc<'gc, ListCell<'gc>>, Gc<'gc, ListCell<'gc>>),
/// A pair of items (car, cdr) where cdr is a list
Cons(Option<Primitive<'gc>>, Gc<'gc, ListCell<'gc>>),