In particular, I have some set S = \{s_1, s_2, s_3, ..., s_n\} and a subset S^\prime, and I want to denote the average of the elements in S^\prime. I would generally just use \frac{\sum\limits_{i=1}^n s_i}{n}, but S^\prime only contains some of the elements of S and so this won't work. This page suggests that the proper notation is \left, but I wasn't able to find this anywhere else. Is this notation common, or is there some other accepted notation that I could use? Thanks!
(This is in a computer science paper, if it makes a difference.)
