If Multiset extends from the Array class
x = [1, 1, 2, 4, 7]
y = [1, 2, 2, 2]
z = [1, 1, 3, 7]
x.union(y) # => [1, 2, 4, 7] (ONLY IN RUBY 2.6)
x.union(y, z) # => [1, 2, 4, 7, 3] (ONLY IN RUBY 2.6)
x | y # => [1, 2, 4, 7]
x.difference(y) # => [4, 7] (ONLY IN RUBY 2.6)
x.difference(y, z) # => [4] (ONLY IN RUBY 2.6)
x - y # => [4, 7]
x & y # => [1, 2]
For more info about the new methods in Ruby 2.6, you can check this blog post about its new features