backed out my old attempt at single_hyp_subst_tac (67cd6ed76446)
It never was totally reliable, and better alternatives now exist (Subgoal.FOCUS).

--- a/src/FOL/hypsubstdata.ML	Wed Sep 29 10:05:44 2010 +0200
+++ b/src/FOL/hypsubstdata.ML	Wed Sep 29 11:02:24 2010 +0200
@@ -13,8 +13,6 @@
   val subst = @{thm subst}
   val sym = @{thm sym}
   val thin_refl = @{thm thin_refl}
-  val prop_subst = @{lemma "PROP P(t) ==> PROP prop (x = t ==> PROP P(x))"
-                     by (unfold prop_def) (drule eq_reflection, unfold)}
 end;
 
 structure Hypsubst = HypsubstFun(Hypsubst_Data);

--- a/src/HOL/HOL.thy	Wed Sep 29 10:05:44 2010 +0200
+++ b/src/HOL/HOL.thy	Wed Sep 29 11:02:24 2010 +0200
@@ -834,8 +834,6 @@
   val subst = @{thm subst}
   val sym = @{thm sym}
   val thin_refl = @{thm thin_refl};
-  val prop_subst = @{lemma "PROP P t ==> PROP prop (x = t ==> PROP P x)"
-                     by (unfold prop_def) (drule eq_reflection, unfold)}
 end);
 open Hypsubst;
 

--- a/src/Provers/hypsubst.ML	Wed Sep 29 10:05:44 2010 +0200
+++ b/src/Provers/hypsubst.ML	Wed Sep 29 11:02:24 2010 +0200
@@ -42,13 +42,10 @@
   val subst            : thm               (* [| a=b;  P(a) |] ==> P(b) *)
   val sym              : thm               (* a=b ==> b=a *)
   val thin_refl        : thm               (* [|x=x; PROP W|] ==> PROP W *)
-  val prop_subst       : thm               (* PROP P t ==> PROP prop (x = t ==> PROP P x) *)
 end;
 
 signature HYPSUBST =
 sig
-  val single_hyp_subst_tac   : int -> int -> tactic
-  val single_hyp_meta_subst_tac : int -> int -> tactic
   val bound_hyp_subst_tac    : int -> tactic
   val hyp_subst_tac          : int -> tactic
   val blast_hyp_subst_tac    : bool -> int -> tactic
@@ -61,27 +58,6 @@
 
 exception EQ_VAR;
 
-val meta_subst = @{lemma "PROP P t \<Longrightarrow> PROP prop (x \<equiv> t \<Longrightarrow> PROP P x)"
-  by (unfold prop_def)}
-
-(** Simple version: Just subtitute one hypothesis, specified by index k **)
-fun gen_single_hyp_subst_tac subst_rule k = CSUBGOAL (fn (csubg, i) =>
- let 
-   val pat = fold_rev (Logic.all o Free) (Logic.strip_params (term_of csubg)) (Term.dummy_pattern propT)
-             |> cterm_of (theory_of_cterm csubg)
-
-   val rule =
-       Thm.lift_rule pat subst_rule (* lift just over parameters *)
-       |> Conv.fconv_rule (MetaSimplifier.rewrite true [@{thm prop_def}])
- in
-   rotate_tac k i
-   THEN Thm.compose_no_flatten false (rule, 1) i
-   THEN rotate_tac (~k) i
- end)
-
-val single_hyp_meta_subst_tac = gen_single_hyp_subst_tac meta_subst
-val single_hyp_subst_tac = gen_single_hyp_subst_tac Data.prop_subst
-
 fun loose (i,t) = member (op =) (add_loose_bnos (t, i, [])) 0;
 
 (*Simplifier turns Bound variables to special Free variables: