One of the most vexing problems for Chicago and its teachers went virtually unmentioned during the strike: The pension fund is about to run dry, the New York Times reported today. The Chicago Teachers’ Pension Fund has about $10 billion in assets, but is paying out more than $1 billion in benefits a year — much more than it has been taking in. That has forced it to sell investments, worth hundreds of millions of dollars a year, to pay retired teachers. Experts say that the fund could collapse within a few years unless something is done.