Parenthsis operator for sets or probability

[mkommar]

It's been more than a couple of years since my formal math classes and I ran across this document:
http://sysbio.oxfordjournals.org/content/45/3/380.full.pdf

The problem I'm having is that I can't seem to remember what the parenthetical operator does in equation 16.

Thanks!

 

[CRGreathouse]

http://mathworld.wolfram.com/BinomialCoefficient.html