Who Cites is leaf?
is_leaf	Def is_leaf(t) == Case(t) Case tree_leaf(l) => true Default => false
	Thm* E:Type, t:Tree(E). is_leaf(t)
case_default	Def Default => body(value,value) == body
case_tree_leaf	Def Case tree_leaf(x) => body(x) cont(x1,z) Def == InjCase(x1; x2. body(x2); _. cont(z,z))
case	Def Case(value) body == body(value,value)

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