[NTG-context] \definemathcommand lowers the text

Hi! look at the output of this (attached) \definemathcommand [D] [nolop] {\mfunction{D}} \starttext Compare $\D f$ with $\mfunction{D}f$. \stoptext In the \D, the D is lowered a bit. I think it should not be, or else I misuse something. If so, how should I define my own operator (I found this example on the wiki). /Mikael