src/HOL/Tools/Function/partial_function.ML

       handle Option.Option => error (cat_lines ["Unknown mode " ^ quote mode ^ ".",
         "Known modes are " ^ commas_quote (known_modes lthy) ^ "."]);
     val Setup_Data {fixp, mono, fixp_eq, fixp_induct, fixp_induct_user} = setup_data;
 
     val ((fixes, [(eq_abinding, eqn)]), _) = prep fixes_raw [eqn_raw] lthy;
-    val ((_, plain_eqn), args_ctxt) = Variable.focus eqn lthy;
+    val ((_, plain_eqn), args_ctxt) = Variable.focus NONE eqn lthy;
 
     val ((f_binding, fT), mixfix) = the_single fixes;
     val fname = Binding.name_of f_binding;
 
     val cert = Thm.cterm_of lthy;