anchor.diff:                                      (frames-per-second *frames-per-second*)
                                       include)
     (ensure-directories-exist output-directory)
  -  (map nil #'delete-file (frame-files output-directory))
  +  (let ((frame-files (frame-files output-directory)))
  +    (if append
  +        (setq frame-start (if frame-files
  +                              (1+ (last-frame-number frame-files))
  +                              0))
  +        (map nil #'delete-file frame-files)))
     ;; Write frames
     (loop
        for frame from frame-start
  
positive.code: 
  (defun frame-files (directory)
    (directory (concatenate 'string directory "/*.pov")))
  
  (defun animate-encode (&key
                           (directory *robray-tmp-directory*)
  
negative.code:                                     (output-directory *robray-tmp-directory*)
                                      ;(pov-file "frame")
                                      (frame-start 0)
                                      append
                                      (scene-graph *scene-graph*)
                                      (width *width*)
                                      (height *height*)
  