Using "on" as modifier

We want to use "on" (or "start_at") as a modifier, since it simplifies scenarios:

Current scenario, in which one wants to do sth when an event starts:

serial(): 
   vehicle.drive() with:
      until my_condition
   vehicle.my_action()

Same with less code and more intuitive

   vehicle.my_action() with:
      start_at my_condition()