# File lib/rubygame/events/joystick_events.rb, line 209
      def initialize( joystick_id, ball, rel )

        unless joystick_id.kind_of?(Fixnum) and joystick_id >= 0
          raise ArgumentError, "joystick_id must be an integer >= 0"
        end

        @joystick_id = joystick_id

        unless ball.kind_of?(Fixnum) and ball >= 0
          raise ArgumentError, "ball must be an integer >= 0"
        end

        @ball = ball

        @rel = rel.to_ary.dup
        @rel.freeze

        unless @rel.length == 2
          raise ArgumentError, "rel must have exactly 2 parts (got %s)"%@rel.length
        end

      end