[NTG-context] The size keyword in \fenced seems not working

The following code (copied from the math manual) doesn’t produce the intended results with the most recent version of ConTeXt: \starttext \startformula \fenced[bracket]{\frac{1 + x}{1 - x}} = \fenced[bracket][size=0]{\frac{1 + x}{1 - x}} = \fenced[bracket][size=1]{\frac{1 + x}{1 - x}} = \fenced[bracket][size=2]{\frac{1 + x}{1 - x}} = \fenced[bracket][size=3]{\frac{1 + x}{1 - x}} = \fenced[bracket][size=4]{\frac{1 + x}{1 - x}} \stopformula \stoptext What's the proper way of setting size in \fenced ? Thanks in advance. FV Leung