Some definitions of interest.
tree	Def Tree(E) == rec(T.tree_con(E;T))
	Thm* E:Type. Tree(E)  Type
tree_leaf	Def tree_leaf(x) == inl(x)
	Thm* E,T:Type, x:E. tree_leaf(x)  tree_con(E;T)
	Thm* E:Type, x:E. tree_leaf(x)  Tree(E)

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