All the formats currently treated allow the use of the following
plain TEX ??w![scratch register conventions]
( [#!K84:TeXbook!#, p.346])
- |@| is available as a local scratch counter register.
- |@|, |@i|, |@ii|, |3| to |9|, and
|255| are available as scratch dimension registers.
- |@|, |1| to |9|, and |255| are available as
scratch skip registers.
- |@| and |1| to |9| are available as scratch token
lists.
- |@ | and |1| to |9| are available as scratch box
registers.
with the constraint that ??c![] should never (always)
be used when assigning to number 0, 2, 4, 6, 8, and 255 (1, 3, 5, 7,
and 9).