[NTG-context] inside and outside margins (vs. 'left' and 'right' margins)

I want to define 'inside' and 'outside' margins (i.e. left and right pages are symmetrical, not the same). Reading the document 'co-pagedesign.pdf', ConTeXt talks about 'left' and 'right' margins (and other measurements). At first I thought this must be either a 'representative' left or right page (the diagram on page 3 doesn't say which), but experimenting suggests that there really is no concept of left and right pages here (despite the incredible detail that the document goes into on arranging pages etc which must understand this). In short this is a basic requirement. How do I solve it? James