anchor.diff:  
   ;;; Classes
   
  -(defclass hipster-town-snapshot ()
  +(defclass town-snapshot ()
     ((ticks :initform 0
             :initarg :time
             :accessor ticks
  
positive.code: (defgeneric deconform (town)
    (:documentation "Simulates the selection of styles of all hipsters.
  In this case, they will attempt at random any style on the less popular half of the style.")
    (:method ((town hipster-town-snapshot))
      (let ((candidate-styles (survey town)))
        (mapcar #'(lambda (style-of-self)
                    (cond
  
negative.code: 
  (defgeneric tick (town)
    (:documentation "Nondestructively returns the next iteration of the simulation.")
    (:method ((town town-snapshot))
      (make-instance 'town-snapshot
                     :time                (1+ (ticks town))
                     :population          (deconform town)
                     :styles              (styles town)
  