def successors(self):
(fu, fv) = self.face
dist = lambda (u,v): math.sqrt((fu-u)*(fu-u) + (fv-v)*(fv-v))
succ = [ self.map.map_state(n)
for n in grid.neighbours2(self.face)
if self.map.valid_face(n) ]
return [ (s, self.map.cost(s.face) * dist(s.face)) for s in succ ]
