boundary char and implicit kerns

Han The Thanh
Fri, 30 Jun 2000 03:08:26 -0400

Dear all,

I sent the following question to tex-implementors list, but didn't get any
answer yet. In case anyone know the answer, I would be much appreciated.

----- Forwarded message from thanh -----

Dear TeX implementors,

does anyone know why implicit kern is not treated as discardable? Under
which circumstances an implicit kern can end up that the beginning or the
end of a line after breaking a paragraph into lines? 

It seems to me that it can happen for kerns with the boundary char. Is this
the only case when it may happen?

Thanks in advance for the answers,

----- End of forwarded message from thanh -----

In particular, I found quite unfortunate that when using a font
containings kerns with the boundary char, character protruding (ie marginal
kerning) stops to work for certain characters. These characters are
preceded/followed by an implicit kern with the boundary char, which is not
discardable. For example, the period or comma char are not protruded
if there one has in an afm something like:

KPX period space 118
KPX comma space 118

and space is set as the right boundary character.

So now the question is: should implicit kerns be treated as