[texhax] Constructing a very thin space

[Karl Berry]

I do not have an example at hand, but without the space after `em', TeX
will still be reading and expanding tokens after the macro is called (to
look for the <space> that's supposed to be there).  Thus I expect
problems could occur with use in \halign templates and \span, and
differences could be found using \expandafter and similar
expansion-control commands.  Knuth did not put that final space in the
definition of \thinspace just for fun (\relax would work too).

It's definitely the best practice to make sure that macros terminate
their arguments.  Otherwise it's just asking for trouble.

Best,
karl