For your information, using the code I mentionned for \Uppercase, I
have now two more things.

- (Expandable) version of \tl_reverse:n that preserves spaces, and
goes into brace groups, namely after two steps,

\UL_reverse:n{\a{ b \) cd\(e}f}  ->  f{e\(dc \) b }\a

- Expandable full-expansion code (which takes care of course of
\unexpanded and \noexpand, and \protected commands).
         \fullyexpand:n{<some tl>}
is then equivalent to
         \edef\tmpa{<some tl>} \tmpa

I'm not too sure where I should go next.

> I know of Uwe Lück's fifinddo.sty code, as part of the nicetext bundle:
>   <>

Interesting. I will take a look.