op_U : ('a -> 'a -> bool) -> 'a list list -> 'a list
Takes the union of a list of sets, modulo the supplied relation.
An application op_U eq [l1, ..., ln] is equivalent to op_union eq l1 (... (op_union eq ln-1, ln)...). Thus, every element that occurs in one of the lists will appear in the result. However, if there are two elements x and y from different lists such that eq x y, then only one of x and y will appear in the result.
If an application of eq fails when applied to two elements from the lists.

- op_U (fn x => fn y => x mod 2 = y mod 2)
       [[1,2,3], [4,5,6], [2,4,6,8,10]];
> val it = [5, 2, 4, 6, 8, 10] : int list

The order in which the elements occur in the resulting list should not be depended upon.

A high-performance implementation of finite sets may be found in structure HOLset.

There is no requirement that eq be recognizable as a kind of equality (it could be implemented by an order relation, for example).

HOL  Kananaskis-14