How to parenthesize a time signature

[Jonathan Wilkes]

Greetings Lilypouncers,
     Is there a quick way to parenthesize a simple time signature?  
I'm not sure whether this is standard or not, but I used it once or 
twice at the beginning of a score that shows an excerpt from, say, the 
middle of a piece, to make it clear that the time signature also 
applied to the music leading up to the excerpt.  (I think I used it a 
long time ago in a copy job for a musicologist, but I can't be sure.)

Thanks,
Jonathan