mutable struct ListItem
    next::ListItem
    prev::ListItem
    function ListItem() 
        x = new()
        x.next = x
        x.prev = x
        x
    end
    function ListItem(n::ListItem, p::ListItem)
        x=new()
        x.next = x
        x.prev = p
        x
    end
    
end

a = ListItem() 
b = ListItem(b, a)
println(b.next == b)
println(b.prev == a)


