[NTG-context] amsl: startcases/stopcases

Hello Giuseppe, I'm having a little trouble with the cases in amsl. Compiling this: \[ f(x) = \startcases 0 & \text{if $x < -\epsilon$} \\ \frac12 & \text{if $x > \epsilon$} \stopcases \] results in an error around the \frac. Changing it to \dfrac makes the input compile ok, but typesets the cases in an ugly manner with really narrow spacing. Is there some way of telling startcases/stopcases to behave in display mode? Thanks in advance, David