And now that we know where our cancel+schedule points are, there’s the question of how to effectively communicate this information to the user. We want some way to mark out a category of functions that might block or trigger a task switch, so that they’re clearly distinguished from functions that don’t do this. Wouldn’t it be nice if there were some Python feature, that naturally divided functions into two categories, and maybe put some sort of special syntactic marking on with the functions that can do weird things like block and task switch…? What a coincidence, that’s exactly how async functions work! Rule 4: in Trio, only the potentially blocking functions are async. So e.g. Event.wait() is async, but Event.set() is sync.