Tidy up 2024 day 11 code.

lib/aoc.ml

@@ -6,6 +6,11 @@ let distance1 a b = abs (a - b)
 let strings_of_file fname =
   In_channel.with_open_text fname In_channel.input_lines
 
+let string_of_file fname =
+  match In_channel.with_open_text fname In_channel.input_line with
+  | Some x -> x
+  | None -> failwith "Aoc.string_of_file"
+
 let main prep parts =
   try
     match Sys.argv with

lib/aoc.mli

@@ -9,6 +9,9 @@ val strings_of_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. *)
 
+val string_of_file : string -> string
+(** [string_of_file fname] returns the first line in [fname]. *)
+
 val log10i : int -> int
 (** [log10i n] returns the floor of [(log10 (float_of_int n))]. [n] must be
     positive. *)