Add char-at to library

lib/lib-advent.org

@@ -20,6 +20,12 @@
       (--map (string-to-char it) (split-string str "\\|.+" t)))
   #+end_src
 
+  #+begin_src emacs-lisp
+      (defun advent/char-at (p l)
+        "return the char at position P in the 2D list l"
+    (nth (car p) (nth (cadr p) l)))
+  #+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