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\DOC op_U \TYPE {op_U : ('a -> 'a -> bool) -> 'a list list -> 'a list} \SYNOPSIS Takes the union of a list of sets, modulo the supplied relation. \KEYWORDS list, set. \DESCRIBE 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. \FAILURE If an application of {eq} fails when applied to two elements from the lists. \EXAMPLE { - 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 } \COMMENTS The order in which the elements occur in the resulting list should not be depended upon. High performance set operations may be found in the ML Standard Basis Library. There is no requirement that {eq} be recognizable as a kind of equality (it could be implemented by an order relation, for example). \SEEALSO Lib.U, Lib.op_mem, Lib.op_insert, Lib.op_union, Lib.op_mk_set, Lib.op_intersect, Lib.op_set_diff. \ENDDOC