anchor.diff:      :initarg :difficulty
       :initform 0)))
   (defclass enemy-bullet (sprite mobile collidable alarm)
  -  ((timer :initform 90)))
  +  ((timer :initform 60)))
   (defclass explosion (sprite mobile collidable alarm)
     ((drop-class
       :initarg :drop-class
  
positive.code: 
  (defmethod alarm ((enemy enemy) event)
    (with-slots (parent offset size veloc timer) enemy
      (setf timer (+ 5 (mt19937:random 35)))
      (shoot enemy event)))
  
  (defmethod collide ((enemy enemy) event)
  
negative.code:     :initarg :missiles
      :initform 0)))
  (defclass bullet (sprite mobile collidable alarm)
    ((timer :initform 60)))
  (defclass missile (sprite mobile collidable alarm)
    ((timer :initform 120)))
  (defclass enemy (sprite mobile collidable alarm shooter)
  