breqn

\eq@capture
\eq@punct
738 \let\\\eq@newline
739 \let\insert\eq@insert \let\mark\eq@mark \let\vadjust\eq@vadjust
740 \hsize\maxdimen \pretolerance\@M
Here it is better not to use \@flushglue (0pt plus1fil) for \rightskip, or else
a negative penalty (such as −99 for \prerelpenalty) will tempt TEX to use
more line breaks than necessary in the first typesetting pass. Ideal values for
\rightskip and \linepenalty are unclear to me, but they are rather sensitively
interdependent. Choice of 10000 pt for rightskip is derived by saying, let’s use a
value smaller than 1 fil and smaller than \hsize, but more than half of \hsize
so that if a line is nearly empty, the glue stretch factor will always be less than
2.0 and so the badness will be less than 100 and so TEX will not issue badness
warnings.
741 \linepenalty\@m
742 \rightskip\z@\@plus\@M\p@ \leftskip\z@skip \parfillskip\z@skip
743 \clubpenalty\@ne \widowpenalty\z@ \interlinepenalty\z@
After a relation symbol is discovered, binop symbols should start including a
special offset space. But until then \EQ@prebin@space is a no-op.
744 \global\let\EQ@prebin@space\relax
Set binoppenalty and relpenalty high to prohibit line breaks after mathbins and
mathrels. As a matter of fact, the penalties are then omitted by TEX, since bare
glue without a penalty is not a valid breakpoint if it occurs within mathon–mathoff
items.
745 \binoppenalty\@M \relpenalty\@M
746 }
If an equation ends with a \right delim, the last thing on the math list will
be a force-break penalty. Then don’t redundantly add another forcing penalty.
(question: when does a penalty after a linebreak not disappear? Answer: when
you have two forced break penalties in a row). Ending punctuation, if any, goes
into the last box with the mathoff kern. If the math list ends with a slanted letter,
then there will be an italic correction added after it by TEX. Should we remove
it? I guess so.
26.1 Capturing the equation
BRM: There’s a problem here (or with \ss@scan). If the LHS has \left
\right pairs, ß@scan gets involved. It seems to produce a separate box marked
w/\penalty 3. But it appears that \eq@repack is only expecting a single box for
the LHS; when it measures that box it’s missing the (typically larger) bracketted
section, so the LHS is measured =¿ 0pt (or very small). I’m not entirely clear
what Michael had in mind for this case; whether it’s an oversight, or whether
I’ve introduced some other bug. At any rate, my solution is to measure the RHS
(accumulated in \EQ@box), at the time of the relation, and subtract that from the
total size.
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