pub enum FreeListCell<'a> {
    /// The empty list
    Null,
    /// A pair of items (car, cdr) where cdr is a list
    Cons(&'a FreePrimitive<'a>, &'a FreeListCell<'a>),

pub struct FreeListCell<'a> {
    /// car of a cons cell
    pub car: Option<&'a FreePrimitive<'a>>,
    /// cdr of a cons cell
    ///
    /// a "list" is when this points to another list or is None
    pub cdr: Option<&'a FreePrimitive<'a>>,
}
impl<'a> FreeListCell<'a> {
    /// Create an empty list cell (Scheme `'()`)
    pub const fn empty() -> Self {
        Self {
            car: None,
            cdr: None,
        }
    }

pub enum ListCell<'gc> {
    /// The empty list
    Null,
    /// A pair of items (car, cdr) where cdr is a list
    Cons(Option<Primitive<'gc>>, Gc<'gc, ListCell<'gc>>),

pub struct ListCell<'gc> {
    car: Option<Gc<'gc, Primitive<'gc>>>,
    /// This cell is a list if this points to another list or is None (unset)
    cdr: Option<Gc<'gc, Primitive<'gc>>>,
}
impl<'gc> ListCell<'gc> {
    /// Create an empty list cell (Scheme `'()`)
    pub const fn empty() -> Self {
        Self {
            car: None,
            cdr: None,
        }
    }