'Adds an event callback so that your application may be notified when a particular event occurs.
Possible events are: "continue", "error", "incomplete" and "hangup".
Argument: event is an event
Argument: **options is a set of optional keyword arguments.
See https://www.tropo.com/docs/webapi/on.htm'

def on(self, event, **options):

    self._steps.append(On(event, **options).obj)
