def update(event, target)
      case event
      when :user_deleted
        @users = @users.delete_if { |element| element == target }
        target.delete_observer(self)
      else
        raise ArgumentError.new(event)
      end
    end
def update(event, target)
      case event
      when :user_deleted
        @users = @users.delete_if { |element| element == target }
        target.delete_observer(self)
      else
        raise ArgumentError.new(event)
      end
    end
observer callback