[XeTeX] using interchartoks to change text direction

[Gareth Hughes]

Could anyone on the list enlighten me how to change back and forth from
LTR to RTL text using \XeTeXinterchartoks. If I define charclass 4 for
my RTL script, I can use bidi commands to change direction to RTL on the
transitions 0→4 and 255→4, and to change back on 4→0. However, the
problem is with the transition 255→0, which could occur in places where
no change of direction is happening (actually 255→4 could also happen in
the middle of a RTL stretch). The question is how can I specify the
exact places where directionality should change.

Thanks,

Gareth.