[NTG-context] \newbibfield[help!] : Very quick question.

Greetings all, In working with the bib module, I've been trying to understand the addition of a field. There is a "\newbibfield" command, but it is not entirely clear where to declare it. E.g., I want to have a field called "yearnote". If I place this in the list of commands in the t-bib.tex file, and call for it with "\insertyearnote" in the \setuppublicationlayout, it works perfectly. But declaring \newbibfield[yearnote] yields me an "undefined control sequence". Is there a special place to declare the latter? — I've tried in my current file, before and after \starttext, in the .bbl file, before and after \setuppublicationlist… no luck! Regards, David