code: def things_near(self, location, radius=None):
          "Return all things within radius of location."
          if radius is None: radius = self.perceptible_distance
          radius2 = radius * radius
          return [thing for thing in self.things
                  if distance2(location, thing.location) <= radius2f things_near(self, location, radius=None):
          "Return all things within radius of location."
          if radius is None: radius = self.perceptible_distance
          radius2 = radius * radius
          return [thing for thing in self.things
                  if distance2(location, thing.location) <= radius2]
docs: Return all things within radius of location.