    # 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