syntax for numbercommand and \gobbleoneargument

[Sanjoy Mahajan <sanjoy@mrao.cam.ac.uk>]

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}