diff --git a/lib/lib-advent.org b/lib/lib-advent.org
index 68e7ad8..e4cd225 100644
--- a/lib/lib-advent.org
+++ b/lib/lib-advent.org
@@ -19,3 +19,16 @@
       "Split the string into a list of chars"
       (--map (string-to-char it) (split-string str "\\|.+" t)))
   #+end_src
+
+  #+begin_src emacs-lisp
+    (defun advent/coordinates-of (e li)
+      "This takes a 2D list (a list of lists) of chars LI and an element E
+    and returns a list of positions in LI that contain the element E.
+    "
+      (-map #'cadr
+            (--filter (eq (car it) e)
+                      (-mapcat #'identity
+                               (-map-indexed (lambda (y l)
+                                               (-map-indexed (lambda (x el) (list el (list x y))) l))
+                                             li)))))
+  #+end_src