Some definitions of interest.
assert	Def b == if b True else False fi
	Thm* b:. b  Prop
is_leaf	Def is_leaf(t) == Case(t) Case tree_leaf(l) => true Default => false
	Thm* E:Type, t:Tree(E). is_leaf(t)
leaf_value	Def leaf_value(t) == Case(t) Case tree_leaf(l) => l Default => True
	Thm* E:Type, t:Tree(E). is_leaf(t)  leaf_value(t)  E
tree	Def Tree(E) == rec(T.tree_con(E;T))
	Thm* E:Type. Tree(E)  Type

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