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