src/Pure/goal.ML

             else err ("Proved a different theorem: " ^ string_of_term (Thm.prop_of res))
           end);
     val res =
       if immediate orelse #maxidx (Thm.rep_cterm stmt) >= 0 orelse not (Future.enabled ())
       then result ()
-      else future_result ctxt' (Future.fork_pri 1 result) (Thm.term_of stmt);
+      else future_result ctxt' (Future.fork_pri ~1 result) (Thm.term_of stmt);
   in
     Conjunction.elim_balanced (length props) res
     |> map (Assumption.export false ctxt' ctxt)
     |> Variable.export ctxt' ctxt
     |> map Drule.zero_var_indexes