src/Pure/thm.ML

 
 fun add_oracle (b, oracle_fn) thy1 =
   let
     val (name, oracles') = Name_Space.define (Context.Theory thy1) true (b, ()) (get_oracles thy1);
     val thy2 = map_oracles (K oracles') thy1;
+    val cert2 = Context.Certificate_Id (Context.theory_id thy2);
     fun invoke_oracle arg =
-      let val ct as Cterm {cert = cert2, t = prop, T, maxidx, sorts} = oracle_fn arg in
+      let val ct as Cterm {cert = cert3, t = prop, T, maxidx, sorts} = oracle_fn arg in
         if T <> propT then
           raise THM ("Oracle's result must have type prop: " ^ name, 0, [])
         else
           let
-            val cert = Context.join_certificate (Context.Certificate thy2, cert2);
+            val cert = Context.join_certificate (cert2, cert3);
             val proofs = Proofterm.get_proofs_level ();
             val oracle =
               if Proofterm.oracle_enabled proofs
               then ((name, Position.thread_data ()), SOME prop)
               else ((name, Position.none), NONE);

               else MinProof;
             val zproof =
               if Proofterm.zproof_enabled proofs then
                 let
                   val thy = Context.certificate_theory cert handle ERROR msg =>
-                    raise CONTEXT (msg, [], [ct], [], SOME (Context.Theory thy2));
+                    raise CONTEXT (msg, [], [ct], [], NONE);
                 in ZTerm.oracle_proof thy name prop end
               else ZDummy;
           in
             Thm (make_deriv [] [oracle] [] [] zproof proof,
              {cert = cert,