As the date approaches September 1st:
-- The probability of a moving truck blocking the sidewalk approaches 1.
-- The size of the trash pile in front of each house approaches infinity.
-- Corollary: the probability of successfully navigating the sidewalk approaches zero.
Okay, it's not a group -- trucks and trash are not sidewalks. But after two glasses of sake, I really strongly think there is a Monadic functor over the function of walking involved here somewhere...