From: Simon Pepping <spepping@scaprea.hobby.nl>
To: Sebastian Rahtz <sebastian.rahtz@computing-services.oxford.ac.uk>
Subject: Re: jadetex 2.8
Date: Mon, 27 Dec 1999 10:29:39 +0100

Sebastian,

Problem with nested displaygroups

Situation: simple-page-sequence with page-n-columns: 2, display-group
with span: 2, display-group.  Here jadetex has a major problem with
the nested display groups:

JadeTeX WARNING: specifying column spans for floats not supported
JadeTeX WARNING: specifying column spans for floats not supported
(\end occurred inside a group at level 10) 

The attached files span.dsl and span.sgml illustrate the problem. The
stylesheet span.dsl offers 11 options, selected by setting the
variable 'selection' to a value between 1 and 11. See the comment in
the style sheet for explanation.

The problem is caused by the fact that a display group with save > 1
ends the multicols environment. The nested display group tries to do
that again. Its end opens a new multicols environment, although none
was closed.

I have tried to rethink the strategy with multicols. One problem is
that the DSSSL spec applies span to page-sequence. While that is not
implemented, it is applied to simple-page-sequence with
page-n-columns, but I have no specification of that.

So I have devised my own scenario:

I have assumed that both display-group and paragraph are to listen to
span.  Only display-groups and paragraphs at top level can change the
span. Every toplevel display-group and paragraph with span > 1 will
start a new multicols environment.

Problem: One could specify span for all children at the
simple-page-sequence level, causing each toplevel display-group and
paragraph to start a new multicols environment. This seems a silly
style specification, so I will not pay attention to it.

That leads me to the following algorithm:

begin:
if attoplevel && pageNcols > 1
  if span > 1
    if newnumcols > 1 // span != pageNcols
      begin multicols // local, open at nesting level 1

end:
if attoplevel && pageNcols > 1
  if span > 1
    if newnumcols > 1 // span != pageNcols
      end multicols // local, close at nesting level 1

Problem: There is always a multicols at toplevel if pageNcols > 1,
started by the simple-page-sequence, but often it will remain
empty. Similarly between successive toplevel paragraphs or
display-groups which specify a span. Solution: Delay starting a
multicols to the actual start of the first paragraph or
display-group. Similarly after a spanned paragraph or display-group.

That modifies the algorithm as follows:

begin:
if attoplevel && pageNcols > 1
  if span > 1
    if inmulticols
      end multicols // toplevel, close at nesting level 0
    if newnumcols > 1 // span != pageNcols
      begin multicols // local, open at nesting level 1
  else if span = 1 && !inmulticols
    begin multicols // delayed opening of toplevel,
                    // toplevel, open at nesting level 0

end:
if attoplevel && pageNcols > 1
  if span > 1
    if newnumcols > 1 // span != pageNcols
      end multicols // local, close at nesting level 1

Problem: What would be expected from a sequence specifying a span for
its children, e.g. the following sosofo:

(make simple-page-sequence page-n-columns: 4
  (make sequence span: 2
    (make paragraph)
    (make paragraph)
    (make paragraph)))

Under the scheme proposed here each paragraph would create its own
multicols environment. Is this the intended behaviour, or should
sequence also listen to span?

Notes about the implementation:

\@displaylevel can have three values:

0: at top level. Display groups and paragraphs which see this value
are at toplevel. Their start may close or open a multicols. They set
the value to 1 after their own \begingroup. Then they may open a
multicols.

1: within a top level display group or paragraph. Their start sets the
value to 2 after their own \begingroup. End of display groups and
paragraphs which see this value may close a multicols.

2: below top level. Display groups and paragraphs which see this value
do nothing with multicols.

The value is automatically decremented by TeX's grouping.

A simple switch \if@toplevel does not do, because it is false when
entering the end of a toplevel display group or paragraph.

\@SavedSpan contains the localized value of the span. It is required
by the end of display groups and paragraphs to see if they should
close a multicols.

The toplevel multicols is closed by the end of a
simple-page-sequence.

The code in ncols.tex goes a long way towards a solution, and it works
in normal cases. But I am very uncertain about the many cases that the
affected \Par definitions serve. Where in the if branches should I
insert my code? For now I just inserted it at a suitable place. See
the lines % SP for my changes.

Note: \Seq may also open a multicols environment. I have not dealt
with it; under the scheme proposed here this possibility might be
removed.