This minimal file [case 1]: \setuphead[chapter][numbercommand={\gobbleoneargument} ] \starttext \chapter{One} \stoptext fails. Whereas if the first two lines are replaced by [case 2]: \setuphead[chapter][numbercommand={\gobbleoneargument}, ] or [case 3]: \setuphead[chapter][numbercommand={\gobbleoneargument}] then it's fine. Is this a bug or a feature of TeX's tricky spacing rules? And how can I think about ConTeXt's parsing in way that the difference between cases 1 and 2,3 is obvious? The error is (with tex : pdfeTeX, 3.141592-1.21a-2.2 (Web2C 7.5.4), context : ver: 2006.04.17 23:14): ERROR: Argument of \gobbleoneargument has an extra }. --- TeX said --- <inserted text> \par <to be read again> } \@@kochapternumbercommand ->{\gobbleoneargument } <argument> ...{\??ko \v!chapter \c!numbercommand } {\setstrut \begstrut \exec... \@@ko::normal ...numbercontent \setbox 0\hbox {{#1 }\hskip \numberheaddistanc... \doplaceheadnumbertext ...utes \dostopattributes } }\fi \endheadplacement {#1... ... l.4 \chapter{One}