new_pos = mover(start_pos)
if new_pos[0] < 0 or new_pos[1] < 0 or \
new_pos[0] >= self.size[0] or new_pos[1] >= self.size[1]:
new_pos = mover(start_pos)
if new_pos[0] < 0 or new_pos[1] < 0 or \
new_pos[0] >= self.size[0] or new_pos[1] >= self.size[1]: