revisions to @Tab package

[Jeff Kingston]

I've been doing some work on the Lout kernel language, adding symbols
that produce arbitrary spanning columns and rows, with a view to adding
these features to @Tab.  I'm interested in hearing from people with
other suggestions for @Tab.  Here is my current list, which I made by
trawling through the @Tab chapter, my memory, and old mailing list
postings (there was a discussion of @Tab in some depth a while ago):


Things that I plan to add
-------------------------

* Horizontal and vertical spanning in arbitrary places in the
  table, specified by @Fmt options and probably also on the fly;

* Vertical centring of table entries (easy right now but needs
  packaging and documentation);

* Cell backgrounds;

* Variable rule thicknesses (at present all rules in @Tab have equal
  thickness).


Things that I am not sure about yet
-----------------------------------

* Fix headings over aligned columns;

* Fix problems with ruled lines not extending the full column width.


Things that would be good but that I don't plan to add
------------------------------------------------------

* Lists that fill multiple columns.  This is arguably quite different
  to @Tab, since @Tab's input format does not apply, although the
  result looks tabular;

* Row splitting (more technically, vertical hyphenation of the rows of
  a table).  This is quite hard, and I'm wimping out this time around;

* Running headers for multi-page tables.  Whereas vertical hyphenation
  has to do with where a table may be split, running headers has to do
  with what happens after the split is made.  It applies at any split
  point, not just hyphenation points, and it applies to any galley, not
  just tables.  I might go for this problem next time I get some time,
  since I have some interesting ideas about it.


Jeff