会社のrubyのバージョンが2.4.0以降にあげられないので `Array#-` と同等のスニペットをおいておく
irb(main):010:0> class Array irb(main):011:1> def - other irb(main):012:2> copy = self.dup irb(main):013:2> other.each do |other_array_item| irb(main):014:3* copy.delete other_array_item irb(main):015:3> end irb(main):016:2> copy irb(main):017:2> end irb(main):018:1> end => :- irb(main):019:0> a = [1, 2, 3] => [1, 2, 3] irb(main):020:0> b = [1, 2, 4] => [1, 2, 4] irb(main):021:0> a - b => [3]
