tuned message: "step" goes back to TTY mode before Proof General, while "depth" is more informative but sometimes confusing due to implementation details;

--- a/src/Pure/Isar/proof.ML	Wed May 07 13:55:16 2014 +0200
+++ b/src/Pure/Isar/proof.ML	Wed May 07 14:05:17 2014 +0200
@@ -365,8 +365,7 @@
       else if mode = Backward then Proof_Context.pretty_ctxt ctxt
       else [];
   in
-    [Pretty.str ("proof (" ^ mode_name mode ^ "): step " ^ string_of_int nr ^
-      (if verbose then ", depth " ^ string_of_int (level state div 2 - 1) else "")),
+    [Pretty.str ("proof (" ^ mode_name mode ^ "): depth " ^ string_of_int (level state div 2 - 1)),
       Pretty.str ""] @
     (if null prt_ctxt then [] else prt_ctxt @ [Pretty.str ""]) @
     (if verbose orelse mode = Forward then