Parenthensize a horizontal group?

[Mark Probert]

Hi.

I've looked through docs and snippets and can't seem to find a way to 
place parens around a horizontal group of notes (an "optional" 
indicator for leadsheets). I've tried 

 \parenthesize { a8 bes }

but nope. Is there a simple way? I want to do something like:

 \mark "(" a8 bes \mark ")"

and have the parens aligned on the center of the staff.

TIA .. mark.