[NTG-context] Opentype features

Hello, I am using Context Mark II + XeTeX. I would like to know the following things. 1. How to use opentype font features. I tried the following code but it is not working. \definefontfeature[myfontfea][+onum] \definetypeface[Libertine][rm][Xserif][Linux Libertine O][default][features=myfontfea] \setupbodyfont[Libertine,12pt] 2. Is there an eqivalent code to the following XeLaTeX code. \defaultfontfeatures{Scale=MatchLowercase} 3. How to use Cambria Math font to typeset math.