Date: 07 Jul 1993 12:45:34 -0400 (EDT)
From: Michael Downes <MJD@MATH.AMS.ORG>
Subject: Around the Bend #9, answer
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To: info-tex@shsu.edu
Reply-to: Michael Downes <MJD@MATH.AMS.ORG>
Message-id: <742063535.36965.MJD@math.ams.org>
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"In internal vertical mode, if the preceding item on the list is a
"vbox, can you do this: \unvbox\lastbox?

The answer is no. If you tried it, you would have seen the error
message:

  ! Missing number, treated as zero.
  <to be read again> 
                     \lastbox 
  l.3   \unvbox\lastbox

  ? h
  A number should have been here; I inserted `0'.
  (If you can't figure out why I needed to see a number,
  look up `weird error' in the index to The TeXbook.)

\lastbox does not return a box register number, which is what \unvbox
requires; instead, \lastbox returns a <box> object in the sense of the
TeXbook, chapter 24, p 278. There are only a few TeX commands that
accept a <box> object as their argument (\shipout, \setbox, \leaders,
...), and \unvbox is not one of them.