% gem install multiset
% irb
> require 'multiset'
> a = Multiset.new([1,1,1,2,3,4])
=> #<Multiset:#3 1, #1 2, #1 3, #1 4>
> b = Multiset.new([1,1,2])
=> #<Multiset:#2 1, #1 2>
> a - b
=> #<Multiset:#1 1, #1 3, #1 4>
irb(main):006:0> (a - b).to_a
=> [1, 3, 4]

うおお