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structure Nat =
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struct
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datatype nat = Dig1 of nat | Dig0 of nat | One_nat | Zero_nat;
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fun plus_nat (Dig1 m) (Dig1 n) = Dig0 (plus_nat (plus_nat m n) One_nat)
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| plus_nat (Dig1 m) (Dig0 n) = Dig1 (plus_nat m n)
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| plus_nat (Dig0 m) (Dig1 n) = Dig1 (plus_nat m n)
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| plus_nat (Dig0 m) (Dig0 n) = Dig0 (plus_nat m n)
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| plus_nat (Dig1 m) One_nat = Dig0 (plus_nat m One_nat)
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| plus_nat One_nat (Dig1 n) = Dig0 (plus_nat n One_nat)
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| plus_nat (Dig0 m) One_nat = Dig1 m
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| plus_nat One_nat (Dig0 n) = Dig1 n
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| plus_nat m Zero_nat = m
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| plus_nat Zero_nat n = n;
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end; (*struct Nat*)
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