diff --git a/foliator.py b/foliator.py
index 90b12c1..a023108 100755
--- a/foliator.py
+++ b/foliator.py
@@ -1,70 +1,71 @@
 #!/usr/bin/env python3
+
+import argparse
 from pypdf import PaperSize, PdfReader, PdfWriter, Transformation
 import math
 
+# Interprets input_pdf as a list of A6 pages, and appends to append_to with
+# A5 pages using the following rules:
+#   a. Odd/even page pairs correspond to front/back pages of a single sheet, printed
+#      with "mirror around **short** edge"
+#   b. when grouped as consecutive sequences of max_folio_size or less, the printed
+#      sheets can be folded in the middle and assembled as a folio
+# FIXME: currently max_folio_size is ignored, and the output is one large folio.
+def foliate_A6(input_pdf, max_folio_size=-1, append_to=PdfWriter()):
+    num_A5 = math.ceil(len(input_pdf.pages)/4.0) * 2
+    writer = append_to
+
+    for i in range(num_A5):
+        dpage = writer.add_blank_page(width=PaperSize.A5.height,
+                                      height=PaperSize.A5.width)
+        ipage = input_pdf.pages[i]
+        ipage.mediabox = dpage.mediabox
+
+        if i % 2 == 0: # Even pages are filled right to left
+            ipage.add_transformation(Transformation().translate(PaperSize.A6.width,
+                                                                0))
+        dpage.merge_page(ipage)
+
+        # Now for the fun part: work backwards to fill the remaining pages
+    for j in range(len(input_pdf.pages) - num_A5):
+        dpage = writer.pages[num_A5 - 1 - j]
+        ipage = input_pdf.pages[num_A5+j]
+        ipage.mediabox = dpage.mediabox
+        if (num_A5 - 1 - j) % 2 == 1:
+            ipage.add_transformation(Transformation().translate(PaperSize.A6.width,
+                                                                0))
+        dpage.merge_page(ipage)
+    return writer
+
+# Take an A5 printing plan, with odd/even pages being understood
+# as front/back pairs of sheets, short edge mirroring. Produces an A4 printing plan
+# containing two A5 panels per sheet, long edge mirroring.
+def blit_to_A4(input_pdf, append_to=PdfWriter()):
+    L = len(input_pdf.pages)
+    writer = append_to
+
+    for k in range(0, L, 2):
+        if(k%4 == 0): # Open up a new sheet
+            front_page = writer.add_blank_page(width=PaperSize.A4.width,
+                                               height=PaperSize.A4.height)
+            back_page = writer.add_blank_page(width=PaperSize.A4.width,
+                                              height=PaperSize.A4.height)
+        front = input_pdf.pages[k]
+        back = input_pdf.pages[k+1] if (k+1<L) else create_blank_page(None, PaperSize.A5.width, PaperSize.A5.height)
+
+        front.mediabox = front_page.mediabox
+        back.mediabox  = back_page.mediabox
+
+        if(k%4 == 0): # Translate to bottom half of page
+            front.add_transformation(Transformation().translate(0, PaperSize.A5.width))
+            back.add_transformation(Transformation().translate(0, PaperSize.A5.width))
+
+        front_page.merge_page(front)
+        back_page.merge_page(back)
+    return writer
+
 input_pdf = PdfReader("to_foliate.pdf")
-num_A5 = math.ceil(len(input_pdf.pages)/4.0) * 2
-
-# First pretend to be printing on A5 paper
-writer = PdfWriter()
-
-for i in range(num_A5):
-    dpage = writer.add_blank_page(width=PaperSize.A5.height,
-                                  height=PaperSize.A5.width)
-    ipage = input_pdf.pages[i]
-    ipage.mediabox = dpage.mediabox
-
-    if i % 2 == 0: # Even pages are filled right to left
-        ipage.add_transformation(Transformation().translate(PaperSize.A6.width,
-                                                                            0))
-    dpage.merge_page(ipage)
-
-
-# Now for the fun part: work backwards to fill the remaining pages
-for j in range(len(input_pdf.pages) - num_A5):
-    dpage = writer.pages[num_A5 - 1 - j]
-    ipage = input_pdf.pages[num_A5+j]
-    ipage.mediabox = dpage.mediabox
-    if (num_A5 - 1 - j) % 2 == 1:
-        ipage.add_transformation(Transformation().translate(PaperSize.A6.width,
-                                                                            0))
-    dpage.merge_page(ipage)
-
-writer.write("output_A5.pdf")
-
-w2 = PdfWriter()
-if(num_A5 % 4 == 2):
-    writer.add_blank_page(width=PaperSize.A5.height,
-                          height=PaperSize.A5.width)
-    writer.add_blank_page(width=PaperSize.A5.height,
-                          height=PaperSize.A5.width)
-    num_A5 += 2
-
-for i in range(0, num_A5-3, 4):
-    dpage_r = w2.add_blank_page(width=PaperSize.A4.width,
-                               height=PaperSize.A4.height)
-
-    dpage_v = w2.add_blank_page(width=PaperSize.A4.width,
-                                height=PaperSize.A4.height)
-    print(i)
-    top_pr = writer.pages[i]
-    top_pv = writer.pages[i+1]
-
-    bot_pr = writer.pages[i+2]
-    bot_pv = writer.pages[i+3]
-
-    top_pr.mediabox = dpage_r.mediabox
-    bot_pr.mediabox = dpage_r.mediabox
-
-    top_pv.mediabox = dpage_v.mediabox
-    bot_pv.mediabox = dpage_v.mediabox
-
-    bot_pr.add_transformation(Transformation().translate(0, PaperSize.A5.width))
-    bot_pv.add_transformation(Transformation().translate(0, PaperSize.A5.width))
-
-    dpage_r.merge_page(top_pr)
-    dpage_r.merge_page(bot_pr)
-    dpage_v.merge_page(top_pv)
-    dpage_v.merge_page(bot_pv)
-
-w2.write("output.pdf")
+a5 = foliate_A6(input_pdf)
+a5.write("output_A5.pdf")
+a4 = blit_to_A4(a5)
+a4.write("output_A4.pdf")