unconceal symbols stemming from inductive_set specifications, which are regular part of user-space specification;
also unconceal corresponding primitive definitions, which are official conversions between predicates and sets

--- a/src/HOL/Tools/inductive_set.ML	Fri Sep 11 21:44:39 2015 +0200
+++ b/src/HOL/Tools/inductive_set.ML	Thu Sep 10 12:52:24 2015 +0200
@@ -469,7 +469,6 @@
 
     (* define inductive sets using previously defined predicates *)
     val (defs, lthy2) = lthy1
-      |> Proof_Context.concealed  (* FIXME ?? *)
       |> fold_map Local_Theory.define
         (map (fn (((c, syn), (fs, U, _)), p) => ((c, syn), ((Thm.def_binding c, []),
            fold_rev lambda params (HOLogic.Collect_const U $