anchor.diff:      :initarg :missiles
       :initform 0)))
   (defclass bullet (sprite mobile collidable alarm)
  -  ((timer :initform 90)))
  +  ((timer :initform 60)))
   (defclass missile (sprite mobile collidable alarm)
     ((timer :initform 120)))
   (defclass enemy (sprite mobile collidable alarm shooter)
  
positive.code:     :initarg :difficulty
      :initform 0)))
  (defclass enemy-bullet (sprite mobile collidable alarm)
    ((timer :initform 90)))
  (defclass explosion (sprite mobile collidable alarm)
    ((drop-class
      :initarg :drop-class
  
negative.code: 
  (defmethod alarm ((enemy enemy) event)
    (with-slots (parent offset size veloc timer) enemy
      (setf timer (+ 5 (mt19937:random 30) (mt19937:random 30)))
      (shoot enemy event)))
  
  (defmethod collide ((enemy enemy) event)
  