Date: 07 Jul 1993 12:45:34 -0400 (EDT)
From: Michael Downes <[email protected]>
Subject: Around the Bend #9, answer
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To: [email protected]
Reply-to: Michael Downes <[email protected]>
Message-id: <[email protected]>
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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.

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