With \at[ref] one can get the pagenumber of a reference. If this reference is not there (maybe in another file, not included at that moment) there is no error given, but two question marks ??. That is of course useful, but what if I WANT an error? Because that is the only fast way to detect typing errors in references. Else I will have ?? at the end in the book, not very nice :) Huseyin