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

About: IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html