A rule expressing how to create a transition. !(p) This class allows rules for identifying future transitions to be expressed. A rule might be written _in many forms: !(ul) !(li)the 16th March !(li)the Sunday on or after the 16th March !(li)the Sunday on or before the 16th March !(li)the last Sunday _in February </ul> These different rule types can be expressed and queried.