CTAN has a new package: iftex

[CTAN Announcements]

This should within a day be at your local mirror.

Jim Hef{}feron
Saint Michael's College

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The following information was provided by our fellow contributor:

Name of contribution: iftex
Version number: 0.1
Author's name: Vafa Khalighi
Location on CTAN: /macros/latex/contrib/iftex
Summary description: Am I running under PDFTeX, or XeTeX, or LuaTeX?
License type: lppl

Announcement text: 
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This very simple package, for both Plain TeX and LaTeX,
defines the \ifPDFTeX, \ifXeTeX, and \ifLuaTeX boolean
for testing whether PDFTeX, or XeTeX, or LuaTeX is being
used for typesetting.

Also provides the \RequirePDFTeX, \RequireXeTeX, and \RequireLuaTeX
command which throws an error if PDFTeX, XeTeX and LuaTeX (respectively)
is not the engine in use.
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This package is located at 
   http://tug.ctan.org/tex-archive/macros/latex/contrib/iftex
.  More information is at
   http://tug.ctan.org/pkg/iftex
(if the package is new it may take a day for that information to 
appear).  We are supported by the TeX Users Group http://www.tug.org .  
Please join a users group; see http://www.tug.org/usergroups.html .