[NTG-context] callback.register

Hallo, I tried the following -------------------------------------- \starttext \startluacode local OldPreLB function MyPreLB(head,gc) texio.write_nl("########### Here I am!") OldPreLB(head,gc) end OldPreLB = callback.find('pre_linebreak_filter') callback.register('pre_linebreak_filter',MyPreLB) \stopluacode Test \stoptext -------------------------------------- but MyPreLB() is not called. What I am doing wrong here? Wolfgang