rdbgui4sasa: don't run anything if not arg is provided, an print a welcome msg

tools/rdbgui4sasa/rdbgui.ml

@@ -321,6 +321,17 @@ let gui str =
 ;;
 
 
+let welcome () =
+  Printf.printf "rdbgui4sasa is a GUI wrapper around rdbg when used with sasa\n";
+  Printf.printf "you can thus replace rdbg by rdbgui4sasa, but sasa *must be* involved in the session\n";
+  Printf.printf "Example: \n";
+  Printf.printf "   rdbgui4sasa -sut \"sasa g.dot\"\n";
+  Printf.printf "   rdbgui4sasa -h\n\n";
+  flush stdout
+
 let _ =
+  let n = Array.length Sys.argv in
+  welcome ();
+  if n = 1 then exit 0;
   ignore (read_stdout ic_stdout);  
   gui (read_line ()) (* read the session choice *)