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datatype t = A | B
val swap = fn x : t => case x of A => B | B => A
datatype u = C of t | D
val out = fn x : u => case x of C y => y | D => A
datatype nat = O | S of nat
val is_two = fn x : nat =>
case x of S (S O) => A | _ => B
val zero_is_two = is_two O
val one_is_two = is_two (S O)
val two_is_two = is_two (S (S O))
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