diff --git a/p9/p9.org b/p9/p9.org
index 67be3a0..4f6663f 100644
--- a/p9/p9.org
+++ b/p9/p9.org
@@ -2,7 +2,7 @@
 
 #+begin_src emacs-lisp :results none
   (with-temp-buffer
-    (insert-file-contents "input")
+    (insert-file-contents "input-test")
     (advent/replace-multiple-regex-buffer
      '(("," . " ")
        ("^" . "(")
@@ -15,7 +15,7 @@
 #+end_src
 
 Find max area
-#+begin_src emacs-lisp 
+#+begin_src emacs-lisp
   (defun area (el)
     (let ((a (car el))
   	(b (cdr el)))
@@ -27,9 +27,196 @@ Find max area
                          (-drop (1+ it-index) list))
                    list)))
 
-  (-max (-map 'area (symmetric-pairs data)))
+   (-max (-map 'area (symmetric-pairs data)))
 #+end_src
 
 #+RESULTS:
-: 4737096935
+: 50
 
+For part 2, we begin by removing the vertices that are not corners.
+(there may be some, or none, but I don't know for sure)
+#+begin_src emacs-lisp :results none
+  (setq data-prev (-rotate 1 data)
+        data-next (-rotate -1 data)
+        data-pv (-zip-lists data data-prev data-next))
+
+  (defun normalize (x)
+    (if (= x 0) 0
+      (/ x (abs x))))
+
+  (defun cornerize (a b)
+    (list (normalize (- (car b) (car a)))
+          (normalize (- (cadr b) (cadr a)))))
+
+  (defun hor-or-ver (a b)
+    (if (= (car a) (car b)) 'vertical
+      'horizontal))
+
+  (setq data-corners
+        (--map (cons (car it)
+                     (cons (cornerize (cadr it) (car it))
+                           (cornerize (car it) (caddr it))))
+               data-pv))
+
+  (setq data-corners
+        (--remove (equal (cadr it) (cddr it)) data-corners))
+#+end_src
+
+OK, the datapoints are all corners. Now I know. Find which way is inside
+#+begin_src emacs-lisp
+      ;; find the leftmost and topmost coordinate
+      (setq leftmost (-min (-map #'car data))
+            topmost (-min (-map #'cadr data)))
+
+    ;; find corners that lie on the leftmost coordinate; the domain must
+    ;; be to their right it appears that there are only two such corners;
+    ;; take the first one, and the outgoing direction; it is going down
+    ;; and it must come from the right, so it should be (-1 0) . (0 -1)
+
+      (car (--filter (= (caar it) leftmost) data-corners))
+
+    ;; We therefore know what corners are convex and what corners are concave
+    (setq quadrant-map
+          '( (((-1 0) . (0 -1)) . (4))
+             (((0 -1) . (1  0)) . (1))
+             (((1 0)  .  (0 1)) . (2))
+             (((0 1)  . (-1 0)) . (3))
+             (((-1 0) . (0 1)) . (2 3 4))
+             (((0 1) . (1  0)) . (1 3 4))
+             (((1 0)  .  (0 -1)) . (4 1 2))
+             (((0 -1)  . (-1 0)) . ( 1 2 3))))
+
+    (setq rects (symmetric-pairs data))
+
+    ;; first filter away those that strictly contain a vertex
+    (defun strictly-contains-p (rect p)
+      (let ((minx (min (caar rect) (cadr rect)))
+            (maxx (max (caar rect) (cadr rect)))
+            (miny (min (cadar rect) (caddr rect)))
+            (maxy (max (cadar rect) (caddr rect)))
+            (px (car p))
+            (py (cadr p)))
+        (and (< minx px) (< px maxx)
+             (< miny py) (< py maxy))))
+
+    (setq rects-sifted (-remove (lambda (rect) (--any (strictly-contains-p rect it) data)) rects))
+#+end_src
+
+#+begin_src emacs-lisp
+    (defun incompatible-p (rect corner)
+      (let ((minx (min (caar rect) (cadr rect)))
+            (maxx (max (caar rect) (cadr rect)))
+            (miny (min (cadar rect) (caddr rect)))
+            (maxy (max (cadar rect) (caddr rect)))
+            (px (caar corner))
+            (py (cadar corner))
+            (quadrant-list (cdr (assoc (cdr corner) quadrant-map))))
+        (or (and (= px minx) (< miny py) (< py maxy) (< 2 (length (-intersection '(1 4) quadrant-list))))   ; on left edge
+            (and (= px maxx) (< miny py) (< py maxy) (< 2 (length (-intersection '(2 3) quadrant-list))))
+            (and (= py miny) (< minx px) (< px maxx) (< 2 (length (-intersection '(3 4) quadrant-list))))
+            (and (= py maxy) (< minx px) (< px maxx) (< 2 (length (-intersection '(1 2) quadrant-list)))))))
+
+    (setq final-rects (-remove (lambda (rect) (--any (incompatible-p rect it) data-corners)) rects-sifted))
+    (-max (-map #'area final-rects))
+#+end_src
+
+#+RESULTS:
+: 40
+
+#+begin_src emacs-lisp
+  (car rects)
+  (--filter (incompatible-p (cadr rects-sifted) it) data-corners)
+
+  (cadr rects-sifted)
+
+#+end_src
+
+#+RESULTS:
+| (7 1)  | 11 | 1 |
+| (7 1)  | 11 | 7 |
+| (7 1)  |  9 | 7 |
+| (7 1)  |  9 | 5 |
+| (7 1)  |  2 | 5 |
+| (7 1)  |  2 | 3 |
+| (7 1)  |  7 | 3 |
+| (11 1) | 11 | 7 |
+| (11 1) |  9 | 7 |
+| (11 1) |  9 | 5 |
+| (11 1) |  2 | 5 |
+| (11 1) |  2 | 3 |
+| (11 1) |  7 | 3 |
+| (11 7) |  9 | 7 |
+| (11 7) |  9 | 5 |
+| (11 7) |  2 | 5 |
+| (11 7) |  2 | 3 |
+| (11 7) |  7 | 3 |
+| (9 7)  |  9 | 5 |
+| (9 7)  |  2 | 5 |
+| (9 7)  |  2 | 3 |
+| (9 7)  |  7 | 3 |
+| (9 5)  |  2 | 5 |
+| (9 5)  |  2 | 3 |
+| (9 5)  |  7 | 3 |
+| (2 5)  |  2 | 3 |
+| (2 5)  |  7 | 3 |
+| (2 3)  |  7 | 3 |
+
+#+begin_src emacs-lisp
+  rects-sifted
+#+end_src
+
+#+RESULTS:
+| (7 1)  | 11 | 1 |
+| (7 1)  |  9 | 7 |
+| (7 1)  |  9 | 5 |
+| (7 1)  |  2 | 5 |
+| (7 1)  |  2 | 3 |
+| (7 1)  |  7 | 3 |
+| (11 1) | 11 | 7 |
+| (11 1) |  9 | 7 |
+| (11 1) |  9 | 5 |
+| (11 1) |  2 | 3 |
+| (11 1) |  7 | 3 |
+| (11 7) |  9 | 7 |
+| (11 7) |  9 | 5 |
+| (11 7) |  2 | 5 |
+| (9 7)  |  9 | 5 |
+| (9 7)  |  2 | 5 |
+| (9 7)  |  2 | 3 |
+| (9 7)  |  7 | 3 |
+| (9 5)  |  2 | 5 |
+| (9 5)  |  2 | 3 |
+| (9 5)  |  7 | 3 |
+| (2 5)  |  2 | 3 |
+| (2 5)  |  7 | 3 |
+| (2 3)  |  7 | 3 |
+
+#+begin_src emacs-lisp
+  final-rects
+#+end_src
+
+#+RESULTS:
+| (7 1)  | 11 | 1 |
+| (7 1)  |  9 | 7 |
+| (7 1)  |  9 | 5 |
+| (7 1)  |  2 | 5 |
+| (7 1)  |  2 | 3 |
+| (7 1)  |  7 | 3 |
+| (11 1) | 11 | 7 |
+| (11 1) |  9 | 7 |
+| (11 1) |  9 | 5 |
+| (11 1) |  2 | 3 |
+| (11 1) |  7 | 3 |
+| (11 7) |  9 | 7 |
+| (11 7) |  9 | 5 |
+| (11 7) |  2 | 5 |
+| (9 7)  |  9 | 5 |
+| (9 7)  |  2 | 5 |
+| (9 7)  |  2 | 3 |
+| (9 7)  |  7 | 3 |
+| (9 5)  |  2 | 5 |
+| (9 5)  |  2 | 3 |
+| (9 5)  |  7 | 3 |
+| (2 5)  |  2 | 3 |
+| (2 5)  |  7 | 3 |
+| (2 3)  |  7 | 3 |