def add(*args)
      args.flatten! # allow [] as a param
      args.each do |arg|
        arg = arg.to_sym if arg.is_a? String
        @set << arg unless @set.include? arg # avoid duplicates
      end
    end
def add(*args)
      args.flatten! # allow [] as a param
      args.each do |arg|
        arg = arg.to_sym if arg.is_a? String
        @set << arg unless @set.include? arg # avoid duplicates
      end
    end
the way to add items to the set.