[XeTeX] Turning off OT features

[David Perry]

I'm fairly new to TeX and XeTeX, but have an extensive background in 
Unicode and OpenType.  (Yay XeTeX!)  This may be a silly question, but I 
haven't found the answer in the various docs I've looked at.

I can turn on OT features easily enough:

\addfontfeature{Style=Historic,Ligatures=Historical}
same silly distant  \quad AA aa AY ay ag al

which gives me long s and funky medieval ligatures when I use the 
appropriate font.  The only way I know to turn the features OFF (so they 
don't affect the entire rest of the document) is to put an extra pair of 
braces around the whole thing, {\addfontfeature . . . al }

This works OK, but is awkward when more than a line or two of text is 
involved.  Is there a better way?  I tried some \begin and \end stuff 
but that didn't work.

Thanks - David