src/HOL/Tools/Function/partial_function.ML

     val ((fixp, mono), fixp_eq) = the (lookup_mode lthy mode)
       handle Option.Option => error (cat_lines ["Unknown mode " ^ quote mode ^ ".",
         "Known modes are " ^ commas_quote (known_modes lthy) ^ "."]);
 
     val ((fixes, [(eq_abinding, eqn)]), _) = prep fixes_raw [eqn_raw] lthy;
-    val (_, _, plain_eqn) = Function_Lib.dest_all_all_ctx lthy eqn;
+    val ((_, plain_eqn), _) = Function_Lib.focus_term eqn lthy;
 
     val ((f_binding, fT), mixfix) = the_single fixes;
     val fname = Binding.name_of f_binding;
 
     val cert = cterm_of (ProofContext.theory_of lthy);