[NTG-context] interlaced overbrace and underbrace

Hello list, is there a better way to interlace overbraces and underbraces like this? \starttext These are the expressions to combine: $$\overbrace{a+b+c}+d \quad a+\underbrace{b+c+d}$$ This is the desired result: $$\overbrace{a+b+c}+d \kern-16mm\underbrace{\hbox to 15mm{ }}$$ \blank {\tfx(the underbrace should move down a little more)} \stoptext TIA, Massi