(define (batch-set-alpha pattern )
   (let* ((filelist (cadr (file-glob pattern 1))))
      (while (not (null? filelist))
         (let* (
                 (filename (car filelist))
                  (image (car (gimp-file-load 1     ;; 1=RUN-NONINTERACTIVE 
                                       filename ;; file name
                                       filename ;; raw-file name 
                           )
                        )
                  )
                  (drawable (car (gimp-image-get-active-layer image)))
               )
               (plug-in-colortoalpha  1      ;; 1=RUN-NONINTERACTIVE
                                 image ;; image object name
                                 drawable ;; drawable layers
                                 '(255 255 255) ;; Color to make alpha (white)
               )
               (gimp-file-save 1     ;; 1=RUN-NONINTERACTIVE 
                           image  ;; image object name
                           drawable ;; Drawable layer to save
                           (string-append filename ".png") ;; filename
                           (string-append filename ".png") ;; raw-filename
               )
               (gimp-image-delete image))
               (set! filelist (cdr filelist))
      )
   )
)