[NTG-context] \rm for greek letters in math mode

I see the third \delta here as italic, where I would have expected it to be in roman, as for the "d" example below. Likewise for all greek letters, I think. Why is that? How can I have these in roman? \starttext \delta $\delta {\rm \delta}$\crlf d $d {\rm d}$ \stoptext Thanks, -- Kate