Fwd: Re: [texhax] Correcting for math-mode kerning before commas and periods?

[Uwe Lück]

Sorry, my objection was invalid, I am _convinced_ that your
"guess" was correct! [continued below]

>At 21:27 15.10.05, Oleg Katsitadze wrote:
>>\unkern does not seem to work here.  I guess this is because
>>of the two-stage process by which the math lists are
>>processed -- when the math list is read by TeX, there is yet
>>no kern after the S in $S$, so \unkern does not have any
>>effect.  And when TeX converts the math list into horizontal
>>list and inserts the kern after the S, there is already no
>>\unkern.
>
>This had come to my mind as well, so I made experiments with
>\showlists. Yet they seem not to agree with that conjecture.

OK, TeX first builds a math list from the "input stream"
according to Chapters 24 and 26. When some \kern<wd>
comes in, a kern of width <wd> is appended to the math list.
When an \unkern follows immediately, that kern of width
<wd> is removed again. When an \unkern comes in that
has not been preceded by a \kern<wd> immediately, it is
just ignored. Thus a math list can contain kerns, but not
\unkern's. However, the kern (which is a kern indeed!)
from the automatic italic correction according to
Appendix G Rules 13, 17, 18f can come in only when the
math list has been finished and is converted into a
horizontal list, and there is no way for an \unkern to
interfere here. Even $\unkern can't, because it is screened
off by the final \mathoff.

Best,
Uwe L.