src/Provers/classical.ML

     end;
 
 (*Duplication of hazardous rules, for complete provers*)
 fun dup_intr th = zero_var_indexes (th RS classical);
 
-fun dup_elim th = 
-    th RSN (2, revcut_rl) |> assumption 2 |> Seq.hd |> 
-    rule_by_tactic (TRYALL (etac revcut_rl))
-    handle _ => error ("Bad format for elimination rule\n" ^ string_of_thm th);
+fun dup_elim th =
+  (case try
+      (rule_by_tactic (TRYALL (etac revcut_rl)))
+      (th RSN (2, revcut_rl) |> assumption 2 |> Seq.hd) of
+    Some th' => th'
+  | _ => error ("Bad format for elimination rule\n" ^ string_of_thm th));
 
 
 (**** Classical rule sets ****)
 
 datatype claset =