Ruby Array Merge Method : How can I merge two `arrays` like a `set` with no duplicates?  I'm looking for a method.  The `pipe` method ('|') is what I want, and it's described in documentation as `Array.union(another_array)` in version `2.5.1`, but it throws an error.

    a = [1,2,3]
    b = [3,4,5]

    p a|b					# => [1, 2, 3, 4, 5] what I want
    puts
    p a & b				# => [3] intersection
    puts
    p a + b				# => [1, 2, 3, 3, 4, 5] this is a push
    puts
    p a.union(b)	# => undefined method `union' for [1, 2, 3]:Array (NoMethodError)

Is there such a written method for a `union`?