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Date:         Fri, 22 Jan 2010 15:51:02 +0100
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From: Philipp Stephani <st_philipp@YAHOO.DE>
Subject: Re: Collision between mathtools.sty and expl3
To: LATEX-L@listserv.uni-heidelberg.de
In-Reply-To:  <4B59B86F.8070007@morningstar2.co.uk>
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Status: R

Am 22.01.2010 um 15:38 schrieb Joseph Wright:

> On 22/01/2010 14:20, Philipp Stephani wrote:
>> Hello,
>> 
>> mhsetup.sty (included by mathtools.sty) defines a few macros that expl3 wants to define anew, resulting in error messages.
>> 
>> \documentclass{minimal}
>> \usepackage{mathtools}% or mhsetup
>> \usepackage{expl3}% error
>> \begin{document}
>> \null
>> \end{document}
>> 
>> This results in the following errors:
>> 
>> % This is a LaTeX bug: check coding!
>> % ! Command name `\if_num:w' already defined! Current meaning: \ifnum.
>> % \msg_kernel_bug:x ...oding!}\tex_errmessage:D {#1}
>> %
>> % l.56 \cs_new_eq:NN \if_num:w
>> %                                        \tex_ifnum:D
>> 
>> % This is a LaTeX bug: check coding!
>> % ! Command name `\if_case:w' already defined! Current meaning: \ifcase.
>> % \msg_kernel_bug:x ...oding!}\tex_errmessage:D {#1}
>> %
>> % l.57 \cs_new_eq:NN \if_case:w
>> %                                        \tex_ifcase:D
>> 
>> % ! This is a LaTeX bug: check coding!
>> % ! Command name `\if_dim:w' already defined! Current meaning: \ifdim
>> %
>> % For immediate help type H<return>.
>> %  ...
>> %
>> % l.169 \cs_new_eq:NN \if_dim:w
>> %                               \tex_ifdim:D
>> 
>> The macros \if_num:w, \if_dim:w, and \if_case:w are defined using \cs_new_eq:NN.  Maybe you want to change this into \cs_set_eq:NN, or fix mhsetup.sty accordingly.
>> 
>> Regards,
>> Philipp
>> 
> 
> Hello Philipp,
> 
> This is because Morten's stuff has up to now used it's own expl3-like stuff internally. The team as a whole can't change things, unless of course Morten deputises the job to someone!

Well, you could simply change the definitions to use \cs_set_eq:NN for the moment.
The main problem with such collisions is that it creates unnecessary work for users or package writers who want to use expl3 or xparse.