[tex-live] Effect of, and restrictions on, %& pragmat

[Philip TAYLOR]

With the announcement yesterday of a Unicode-based implementation
of WinEDT, I am now trying to migrate back from TeXworks to WinEDT,
which was for years my TeX editor of choice.

But it is now so long since I last used WinEDT that I no longer
remember the full and gory details of the "%&" pragmat that can
be used in the first line of a TeX file to specify how it is
to be processed.  May I therefore ask (in the context of TeX Live 2011),
whether :

1) %& processing is enabled by default;
2) if not, how to enable it;
3) its limitations : can it indicate only a format,
	or can it also indicate an engine ?  In
	other words, can I write :

		%& XeTeX

	then launch TeX on the resulting file yet still
	have it processed by XeTeX ?
4) where such things are documented ?

Philip Taylor