(eplain.info)\ifundefined

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Enter node , (file) or (file)node

7.6.6 '\ifundefined'
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'\ifundefined{CS} T \else F \fi' expands the T text if the control
sequence '\CS' is undefined or has been '\let' to '\relax', and the F
text otherwise.

   Since '\ifundefined' is not a primitive conditional, it cannot be
used in places where TeX might skip tokens "at high speed", e.g., within
another conditional--TeX can't match up the '\if''s and '\fi''s.

   This macro was taken directly from 'The TeXbook', page 308.