Nested Definition of UDT

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I'm changing classes in my project to UDTs (Type-Defs) and I'm running into a problem that I haven't noticed before:

In the class, we can design it like this:

Class cNode
    Parent As cNode
    FirstSibling As cNode
    LastSibling As cNode
End Class

However, we don't seem to be able to do such nesting in UDTs (Type-Defs), for example:

Type tNode
    Parent As tNode
    FirstSibling As tNode
    LastSibling As tNode
End Type

The above definition will show an error. I would like to know, what is a good solution for such a situation? Thanks.