src/HOL/Tools/Function/function_context_tree.ML

   map (fn c => c RS eq_reflection) [@{thm "cong"}, @{thm "ext"}]
 
 (* Called on the INSTANTIATED branches of the congruence rule *)
 fun mk_branch ctxt t =
   let
-    val ((params, impl), ctxt') = Variable.focus t ctxt
+    val ((params, impl), ctxt') = Variable.focus NONE t ctxt
     val (assms, concl) = Logic.strip_horn impl
   in
     (ctxt', map #2 params, assms, rhs_of concl)
   end