[luatex] Info on direction primitives/implementation

[Arthur Reutenauer]

> Reading the LuaTeX manual again, I spot section 9.4 ('Changes from ALEPH
> RC4') which gives a few more clues, at least with regard to directions
> supported (Latin, Arabic, CJK, Monoglian only). So perhaps I need an
> Aleph manual too :-)

  No, Aleph didn't introduce changes to Omega's typesetting direction
model.  It mostly fixed bugs and added a few convenience functions,
which is why its code base was using as LuaTeX's starting point, rather
than Omega's, when the big merge was done (pdfTeX + Omega / Aleph +
Lua).  The changes documented in the manual are from Aleph to LuaTeX:
the latter restricted the possible writing directions to the ones that
are actually used in typeset texts, disabling such fanciful options as
boustrophedon (OK, I'm kidding, that one never was supported ;-)

	Arthur