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structure HOL =
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struct
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type 'a eq = {eqop : 'a -> 'a -> bool};
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fun eqop (A_:'a eq) = #eqop A_;
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end; (*struct HOL*)
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structure List =
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struct
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fun foldr f (x :: xs) a = f x (foldr f xs a)
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| foldr f [] a = a;
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fun member A_ x (y :: ys) =
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(if HOL.eqop A_ y x then true else member A_ x ys)
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| member A_ x [] = false;
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end; (*struct List*)
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structure Set =
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struct
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datatype 'a set = Set of 'a list;
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val empty : 'a set = Set [];
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fun insert x (Set xs) = Set (x :: xs);
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fun uniona xs (Set ys) = List.foldr insert ys xs;
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fun member A_ x (Set xs) = List.member A_ x xs;
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fun unionaa (Set xs) = List.foldr uniona xs empty;
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end; (*struct Set*)
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