Added thin_tac to signature; previous change was useless

--- a/src/Pure/tactic.ML	Thu Sep 05 18:28:54 1996 +0200
+++ b/src/Pure/tactic.ML	Thu Sep 05 18:29:43 1996 +0200
@@ -77,6 +77,7 @@
   val subgoal_tac: string -> int -> tactic
   val subgoals_tac: string list -> int -> tactic
   val subgoals_of_brl: bool * thm -> int
+  val thin_tac: string -> int -> tactic
   val trace_goalno_tac: (int -> tactic) -> int -> tactic
   end;