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A little m4 puzzle

From: Doug McIlroy
Subject: A little m4 puzzle
Date: Sat, 10 Nov 2018 13:01:40 -0500
User-agent: Heirloom mailx 12.5 7/5/10

Consider this predicate on strings.


eq(x,y) yields T or F according as strings x and y are
the same or different. (The strings may contain only
characters allowed in macro names).

The puzzle: If you omit inner quotes from the definition,
eq(A,A) yields T as intended. But what happens when you
repeat the test?

(Incidentally, you can replace T by `$3' and F by `$4'
to make a near replacement for the builtin ifelse.)

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