Produces the set union of that sequence and another.

// 5, 3, 9, 7, 8, 6, 4, 1, 0
Enumerable.create(5, 3, 9, 7, 5, 9, 3, 7)
          .union([8, 3, 6, 4, 4, 9, 1, 0]);

With equality comparer

// 5, 3, 9, 7, 8, 6, 4, 1, 0, '5'
Enumerable.create(5, 3, 9, 7, 5, 9, 3, 7)
          .union([8, 3, 6, 4, 4, 9, 1, 0, '5'],
                 'x, y => x === y');