[NTG-context] Problem with \button

I have found the following somewhere in the documentation. \button{...}[the chapter on whatever] \button{...}[otherdoc::some topic] \button{...}[previouspage] \button{...}[PreviousJump] I have tried them : all run very well except \button{...}[otherdoc::some topic] How can I replace some topic to indicate a chapter in otherdoc or a page ? Thank you.