public void RenderPeriod()
{
var p = new Period { Start = "200401" };
Assert.AreEqual("[200401,∞]", p.ForDisplay());
p.End = "20050212";
Assert.AreEqual("[200401,20050212]", p.ForDisplay());
p.Start = null;
Assert.AreEqual("[∞,20050212]", p.ForDisplay());
}