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From: Stefan Monnier <monnier@iro.umontreal.ca>
Newsgroups: gmane.emacs.help
Subject: Re: empty-line-p
Date: Fri, 30 Mar 2007 01:36:00 -0400
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>  (if (looking-at "^[ \t\f\r]*$")
>      t
>    nil)))

  (if <foo> t nil)

is an eta-regexp, which can be advantageously reduced to just <foo>.


        Stefan


PS: The classical eta-regexp is (lambda (x) (<foo> x)), which is equivalent to
just <foo>.  Another variant is (cons (car <foo>) (cdr <foo>)), which
eta-reduces to just <foo>.