Improved white spaces in usage message (the printed message, the Arg code pretty much sucks)

main.ml

@@ -62,29 +62,32 @@ let main () =
     let rec upto_usage = function
         | [] -> ""
         | (id, _, _, msg) :: tl ->
-            "\n" ^ repeat " " 8 ^ expand 10 id ^ ": " ^ msg ^ (upto_usage tl)
+            "\n" ^ repeat " " 12 ^ expand 10 id ^ ": " ^ msg ^ (upto_usage tl)
     in
     let args_spec = [
+        ("<file>", Arg.Rest (fun s -> ()),
+                 "   Optional input file (default is to read from stdin)");
+
         ("-o", Arg.String (fun s -> args.outfile <- Some s),
-            "Output file (defaults to foo.s for foo.cvc)");
+             "<file> Output file (defaults to foo.s for foo.cvc)");
 
         ("-v", Arg.Int (fun i -> args.verbose <- i),
-            "Set verbosity (0: nothing, 1: phase titles, 2: intermediate, 3: debug)");
+             "<num>  Set verbosity (0: nothing, 1: phase titles, 2: intermediate, 3: debug)");
 
         ("-nocpp", Arg.Unit (fun _ -> args.cpp <- false),
-            "Disable C preprocessor");
+                 "   Disable C preprocessor");
         ("-cpp", Arg.Unit (fun _ -> args.cpp <- true),
-            "Enable C preprocessor (overwrite earlier -nocpp)");
+               "     Enable C preprocessor (overwrite earlier -nocpp)");
 
         ("-noopt", Arg.Unit (fun _ -> args.optimize <- false),
-            "Disable optimization");
+                 "   Disable optimization");
         ("-opt", Arg.Unit (fun _ -> args.optimize <- true),
-            "Enable optimization (overwrite earlier -nocpp)");
+               "     Enable optimization (overwrite earlier -nocpp)");
 
         ("-upto", Arg.String (fun s -> args.endphase <- s),
-            "Stop after the specified phase, and print the intermediate " ^
-            "representation to stderr.\n        " ^
-            "Possible options are:" ^ upto_usage phases);
+                "<phase> Stop after the specified phase, and print the intermediate " ^
+                "representation to stderr.\n        " ^
+                "    Possible options are (in order of execution):" ^ upto_usage phases);
     ] in
 
     let usage =