Inheriting from a type containing instances of itself

[lscrd]

I don’t find this surprising either. There is no reason for children of 
ExtNodes to be restricted to ExtNode. An ExtNode is a Node and, so, its 
children are Nodes which may be but are not required to be ExtNodes.

Node is defined to contains instances of Nodes, not of instance of itself. We 
would need a special syntax to express `itself`.