src/Pure/Syntax/syntax.ML

     val pts = parse gram root' (tokenize lexicon xids str);
 
     fun show_pt pt = writeln (str_of_ast (pt_to_ast (K None) pt));
   in
     if length pts > ! ambiguity_level then
-      (writeln ("Warning: Ambiguous input " ^ quote str);
+      (warning ("Ambiguous input " ^ quote str);
        writeln "produces the following parse trees:";
        seq show_pt pts)
     else ();
     map (pt_to_ast (lookup_trtab parse_ast_trtab)) pts
   end;