Quoting Oliver Buerschaper
Wolfgang, Jelle,
thanks for your suggestions ... problem solved!
This thread brings up something that I have been thinking of for a while. Similar problems occur in amsmath aligned family which accepts an optional argument. mathtools.sty has an option so that the optional argument will work only if placed on the same line. In terms of ConTeXt that means, \startformula[9pt] math \stopformula \startformula [9pt] math \stopformula will put the formula in 9pt font, while \startformula [9pt] math \stopformula will display "[9pt] math". I have not looked into the details of how mathtools implements this, but it should be possible to port it to context. Will something like this be useful to have, or is adding \relax once in a while alright? Aditya