Unable to cancel a DoFn Timer

The following was raised on [Stackoverflow|https://stackoverflow.com/questions/55875832/how-to-cancel-unset-a-dofn-defined-timer]:

Having read Timely (and Stateful) Processing with Apache Beam and looked at the JavaDoc for a Timer, I have successfully setup a timer to be fired. However, I am missing how to request that a timer be cancelled before it has fired. Is this a semantic that is possible / allowable in Beam architecture?

My back-story is that I wish to register a timer to fire a callback if something has not occurred ... but if it HAS occurred prior to the timer firing, then I wish to cancel the timer from being fired.