> << currently involved with programming a footnote package, I was trying
 > to get a clue about just when to insert \color@begingroup and
 > \color@endgroup >>
 > Some years ago, you discussed misfeatures of the TeX 3
 > '\mark'  mechanism and ideas for fixing it.  Are those ideas
 > adequate for David Kastrup's present purposes?

i'm not sure what you are precisely referring with respect to marks at to be
honnest, but i doubt it. the problem wih footnotes is that they split apart
boxes without a decent way to interact with the mechanism. that is okay if
\mark's are concerned (or specials if they simply mark a point in the stream
but fails miserably if the special is trying to implement a stack of regions