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Date: Wed, 5 Sep 2012 09:12:07 +0100
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From: Joseph Wright <[log in to unmask]>
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Hello all,

The \tl_tail:w function is deprecated and will be removed from expl3
after 2012-12-31.

The background here is  that as implemented in the current release,
\tl_tail:n will strip the braces in output such as

  \tl_tail:n { a { b } }

but not

  \tl_tail:n { a { b } { c } }

An upcoming CTAN release will fix the behaviour of \tl_tail:n such that
braces will never be removed (the 'tail' will be 'what is left after
\tl_head:n has acted'). However, this cannot be fixed in \tl_tail:w, and
so we are removing it as documented.

The recommended replacement is

  \use_none:n <tokens to have head removed>

with either inside a \tl_if_blank:nF test or the use of an empty group

  \use_none:nn  <tokens to have head removed> { }

Also note that \tl_tail:n will prevent expansion of the result inside a
x-type expansion

  \tl_set:Nx { \tl_tail:n { <tokens> } }

and expands correctly inside an f-type expansion

  \tl_set:Nf { \tl_tail:n { <tokens> } }

and so the functionality of \tl_tail:w is very rarely required.
Joseph Wright