src/Pure/Tools/dump.scala

       "O:" -> (arg => output_dir = Path.explode(arg)),
       "R" -> (_ => requirements = true),
       "X:" -> (arg => exclude_session_groups = exclude_session_groups ::: List(arg)),
       "a" -> (_ => all_sessions = true),
       "d:" -> (arg => dirs = dirs ::: List(Path.explode(arg))),
+      "g:" -> (arg => session_groups = session_groups ::: List(arg)),
       "l:" -> (arg => logic = arg),
-      "g:" -> (arg => session_groups = session_groups ::: List(arg)),
       "o:" -> (arg => options = options + arg),
       "s" -> (_ => system_mode = true),
       "v" -> (_ => verbose = true),
       "x:" -> (arg => exclude_sessions = exclude_sessions ::: List(arg)))