It has taken me a while to get to grips with \tl_set_rescan:Nnn <tl var> 
{setup} {tokens}, not least the fact that *omission* from the setup 
means "revert to usual catcode". I think this is worth documenting in 
interface3.pdf, since it seems not unreasonable (at least it did to me) 
to suppose that if one has explicitly changed a catcode using 
\tl_set_rescan:Nnn, only a similarly explicit change would revert the 
catcode to its usual value. In particular it would be helpful to 
document the fact that using an empty setup { } reverts everything to 
usual values.

Andrew

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