[XeTeX] Unicode space characters

[Arthur Reutenauer]

> Thanks. If you make a package for this, it might be good to support a  
> more complete set of the various fixed-width "space" characters from  
> the U+200x row;

  Agreed.  It would be straightforward and coherent.  And let's not
forget the narrow no-break space, U+202F :-)

  Incidentally, I'm not sure we want to mess with the zero-width non
joiner, given its importance in Indic scripts.  It seems to me that
processing it as a macro would do more harm than good in these
scenarios, and that we had better let the font take care of it.  For its
uses in other contexts (in particular Persian, as I understand it), the
definition proposed by Tomáš is probably the right one, but for Indic
scripts it really seems dangerous to not handle it as a plain character.

> One question to think about is what they should do if they occur in  
> vertical mode; should they act like "printable" characters,  
> automatically starting a paragraph, or should they act like spaces and  
> simply disappear?

  Interesting point.  I have absolutely no opinion about that :-)

	Arthur