diff --git a/demo.scm b/demo.scm
index e49b0e5..6d3a48e 100644
--- a/demo.scm
+++ b/demo.scm
@@ -7,4 +7,4 @@
 (define graph1 (generate-web 10 10))
 (define graph2 (remove-rect (copy-tree graph1) 10 3 3 4 5))
 (assq-set! graph2 0 (set-node-color (assq-ref graph2 0) red))
-(write-graphs-to-file (list graph1 graph2) "graph.gif")
+(write-graphs-to-file (vector graph1 graph2) "graph.gif")
diff --git a/graphgif.scm b/graphgif.scm
index f69b7e8..bee75f5 100644
--- a/graphgif.scm
+++ b/graphgif.scm
@@ -2,7 +2,8 @@
 
 (use-modules (cairo)
 	     (srfi srfi-1)
-	     (srfi srfi-9 gnu))
+	     (srfi srfi-9 gnu)
+	     (srfi srfi-43))
 
 (re-export (cairo-pattern-create-rgb . create-color))
 
@@ -141,16 +142,16 @@
 
 (define (output-to-file surfaces filename)
   (define pngdir (mkdtemp "/tmp/graphgif_XXXXXX"))
-  (do ([i 1 (1+ i)]
-       [surfaces surfaces (cdr surfaces)])
-      ((null? surfaces))
-    (cairo-surface-write-to-png
-     (car surfaces)
-     (string-append pngdir "/img-" (number->string i) ".png")))
+  (vector-for-each
+   (lambda (i surface)
+     (cairo-surface-write-to-png
+      surface
+      (string-append pngdir "/img-" (number->string i) ".png")))
+   surfaces)
   (system* "ffmpeg" "-i" (string-append pngdir "/img-%d.png") "-r" "1" filename))
 
 (define-public (write-graphs-to-file graphs filename)
-  (output-to-file (map draw-abstract-graph graphs) filename))
+  (output-to-file (vector-map (lambda (_ graph) (draw-abstract-graph graph)) graphs) filename))
 
 ;; Local Variables:
 ;; geiser-scheme-implementation: guile