document.addEventListener('DOMContentLoaded', function(){
  var screen = document.querySelector('.screen');
  sprite_layer(screen, {num_sprites: 5, xspeed: -10});
  sprite_layer(screen, {num_sprites: 5, xspeed: -20});
  sprite_layer(screen, {num_sprites: 2, xspeed: -30});
  sprite_layer(screen, {num_sprites: 1, xspeed: -40, sprite_class:'turtle'});
  sprite_layer(screen, {num_sprites: 1, xspeed: -75, sprite_class:'bullet'});
  sprite_layer(screen, {
      num_sprites: 1, 
      xspeed: 75, 
      yspeed: -100, 
      sprite_class:'mario-raccoon'
  });
});