anchor.diff:  
   (defmethod alarm ((enemy enemy) event)
     (with-slots (parent offset size veloc timer) enemy
  -    (setf timer (+ 2 (mt19937:random 25)))
  +    (setf timer (+ 5 (mt19937:random 35)))
       (shoot enemy event)))
   
   (defmethod collide ((enemy enemy) event)
  
positive.code:    (difficulty
      :initarg :difficulty
      :initform 0)))
  (defclass enemy-bullet (sprite mobile collidable transient) ())
  (defclass explosion (sprite mobile collidable alarm)
    ((drop-class
      :initarg :drop-class
  
negative.code:     :accessor missiles
      :initarg :missiles
      :initform 0)))
  (defclass bullet (sprite mobile collidable alarm)
    ((timer :initform 90)))
  (defclass missile (sprite mobile collidable alarm)
    ((timer :initform 120)))
  (defclass enemy (sprite mobile collidable alarm shooter)
    ((timer :initform 10)
     (bullet-class :initform 'enemy-bullet)
  