Add day 2402.

lib/aoc.mli

@@ -0,0 +1,10 @@
+val nums_from_string : string -> int list
+(** [nums_from_string s] takes a string of space separated integers and gives
+    back a list of the integers. *)
+
+val distance1 : int -> int -> int
+(** [distance1 a b] returns the absolute difference between [a] and [b]. *)
+
+val strings_from_file : string -> string list
+(** [strings_from_file fname] returns a list of strings from the file 
+    [fname].  Each string represents a line from the file. *)