add transfer rule for Let

--- a/src/HOL/Transfer.thy	Thu Apr 19 19:32:30 2012 +0200
+++ b/src/HOL/Transfer.thy	Thu Apr 19 19:36:09 2012 +0200
@@ -234,6 +234,9 @@
 lemma If_parametric [transfer_rule]: "(op = ===> A ===> A ===> A) If If"
   unfolding fun_rel_def by simp
 
+lemma Let_parametric [transfer_rule]: "(A ===> (A ===> B) ===> B) Let Let"
+  unfolding fun_rel_def by simp
+
 lemma comp_parametric [transfer_rule]:
   "((B ===> C) ===> (A ===> B) ===> (A ===> C)) (op \<circ>) (op \<circ>)"
   unfolding fun_rel_def by simp