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datatype
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'a aexp = IF ('a bexp) ('a aexp) ('a aexp)
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| Sum ('a aexp list)
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| Diff ('a aexp) ('a aexp)
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| Var 'a
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| Num nat
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and 'a bexp = Less ('a aexp) ('a aexp)
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| And ('a bexp) ('a bexp)
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| Neg ('a bexp)