U : ''a list list -> ''a list
- U [[1,2,3], [4,5,6], [1,2,5]]; > val it = [3, 6, 4, 1, 2, 5] : int list
A high-performance implementation of finite sets may be found in structure HOLset.
ML equality types are used in the implementation of U and its kin. This limits its applicability to types that allow equality. For other types, typically abstract ones, use the ‘op_’ variants.