> I played a bit, see attachment. Surely Hans will want to improve on this 
> interface, so don't patch any of the core files just now.

  Fantastic!

  Now I played a bit with your file myself, and compared with the
behaviour of an OTP which has the same action: you can see that macros
arguments between square brackets are preserved by OTP, whereas your
function (obviously) converts everything unconditionally. How difficult
would it be to program the same behaviour, that is, make
collectors.handle pass to convert_babel only contiguous ranges of
characters that are situated outside matching brackets?