Some definitions of interest.
subtype	Def S  T == x:S. x  T
tree	Def Tree(E) == rec(T.tree_con(E;T))
	Thm* E:Type. Tree(E)  Type
tree_con	Def tree_con(E;T) == E+(TT)
	Thm* E,T:Type. tree_con(E;T)  Type
tree_node	Def tree_node(x) == inr(x)
	Thm* E,T:Type, x:(TT). tree_node(x)  tree_con(E;T)
	Thm* E:Type, x,y:Tree(E). tree_node(<x,y>)  Tree(E)

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